"Native [language, etc.]" is not a technical term, and it's more of a general description.
In the strictest sense, a native speaker is a monolingual who has spoken the language from birth, continuously, for their whole lives. Maybe even an adult. And maybe even an educated individual.
But it depends on who is using the term. Almost all linguists would agree that a native speaker does not need to be educated. Some might also allow for "heritage" languages, for individuals who have now lost some ability (or never had full knowledge) in a language, but who learned it as a child, usually from their parents. (For example, a child of Chinese immigrants in an English speaking country-- the child would know and use some Chinese for a few years then during school learn and use English and as an adult maybe forget almost all knowledge of Chinese.)
And certainly children can be considered [child] native speakers of a language.
But then there's the question of an age. The general distinction is between child and adult. Children became native speakers. Adults do not. But at what age is this distinguished? Unclear. The idea of a "critical period" for language learning is that at some point, something is different about children who grow up and become adults (biological? behavioral? neurological? social?) and they can no longer learn a language in the same way and do not become native speakers. The critical period is said to begin around 2-3 years of age (no infants are fully competent native speakers of a language yet, and are arguably not capable of learning a language fully yet), and it is said to end some time around puberty (about 12) or maybe a little earlier (9-10). Some studies have said as late as 14-15, and others have said as early as 5. It depends on the domain as well: phonology (sounds) might be earlier than syntax ('grammar' in the traditional sense). And it has also been pointed out that at least a few things (such as in syntax, some particularly complicated kinds of sentences) are not developed until up to age 15, although by age 5-10 children are fully competent users of the language for general purposes. And of course vocabulary learning (often associated with education, at least for some domains of vocabulary) continues all the way through adulthood. But vocabulary itself isn't generally considered part of "native speaker" knowledge.
So the answer might be that around age 5 some things become harder to learn, at least in some individuals. So starting to learn a language after 5 years of age means it would be harder to learn and you might not easily develop all of the native speaker competence that children learning the language do. I've met speakers who learned a language as late as about 14 years and are to my trained linguist ears completely competent and "native" speakers. But that might vary by what you mean as "native". And that's somewhat rare anyway. There have been a few adults (as many as 5% in some studies) reported to test like native speakers, but there are other studies that find that children who started learning a language as "late" as 3 years can actually be tested and shown to not be equivalent to "real" native speakers who started learning the language from birth (there are a few studies arguing for this perspective from researchers in Sweden who have tested Swedish-born speakers and children of immigrants who came to Sweden at around age 3).
A "simultaneous" bilingual, in the most prototypical sense, someone who learned two native languages from birth. So it's both native in the sense of "as a child", and also simultaneous in that they were both learned at the same time. Because of that, it seems to me that your age of 3 is about as old as would make sense: if you started learning a second language at or after age 3, you would be a sequential bilingual, not simultaneous. But you're definitely a simultaneous bilingual from what you said.
A sequential bilingual is someone who could be considered a native (or almost native) speaker of two languages, from early childhood. The most usual case is through school. So a child might speak Spanish at home and then learn English at school starting at age 3 or 5. A language learned AFTER 5 years of age might not really be considered a case of sequential bilingualism because it's getting beyond what would be considered a "native" language by some definitions.
So in your case I would say that you are a simultaneous bilingual (of which languages?

) and speak a third (English) fluently since childhood.
The
technical term for complete fluency is
nativelike. So you can say that you have
nativelike English ability if most people think you speak it natively, or if, in general, there's no practical difference between you and a native speaker. Nativelike is fluent plus actually behaving like a native speaker. For example, you don't just have an ability with the language, but it's easy and natural for you. Being very strongly "nativelike" would actually mean you're almost becoming a native speaker. But by definition, adults (and maybe late childhood learners) can't become native speakers, so we use the term "nativelike".
But that's being very picky. I'd personally say it's fine to call yourself trilingual and a native speaker of three languages, with the first two being simultaneous and third being sequential. But that might not match technical definitions or cutoffs in formal research on these topics. Another area of complication is whether you show any signs of attrition (loss of ability) in the other two languages, and then questions of dominance. If you speak mostly English, are you still a fully competent native speaker of the other two? Do you feel comfortable in all domains? Is your formal education only in one (maybe English), so you don't know words for, for example, advanced mathematics, in the other languages? Does that actually matter? Again, it depends on definitions. But by most definitions you can't "lose" being a native speaker, even if you are no longer actively using (or have some attrition) in the other languages. It's complicated. And some people, at least for some kinds of linguistic research, would see monolingual native speakers as the ideal examples of native speakers. For example, if you're doing a psycholinguistic experiment measuring some kind of processing in the brain. Even if we consider bilinguals to be equal in linguistic ability, the other language(s) might interfere, so they might be excluded from the study.
An important point I've hinted at above is that actual ability and whether or not someone is "native" are not necessarily related. In fact, in some cases, they're completely different things. In the ideal/prototypical situation, a native speaker is a normal/perfect speaker, but there can be lots of variation from that for technical reasons and due to which definition you use.
Sorry for the possibly too long answer, but feel free to ask if you're wondering about any of those details.