Your child sounds like he could've been me as a child. I had exactly the same problem, and the same way of thinking about it. I didn't know how to go to sleep, and my parents largely couldn't help me. I just knew that lights-out was a certain time, but when lights-out happened I usually would lay in bed for an hour or more.
For me, I didn't really figure it out until I had kids myself; until then I would usually have to exhaust myself in order to go to sleep, or read a book in bed until I fell asleep.
My oldest has this problem also, and we've tried to be more strategic about it than my parents were. We talk to him about "sleep strategies," such as making up stories in his head, but we also tell him it's okay to just lay there until he drifts off - there's nothing wrong with it. We also focus on giving him a lot of agency over bedtime; he goes "to bed" well before we really expect him to be asleep, and he is able to read for an hour or more without being over the "needs to be asleep" limit.
I'd encourage you to talk to your three year old about strategies for falling asleep. It's possible he does need a later bedtime, though it sounds like you're still doing okay right now with sleep - so maybe this is just more of an academic exercise right now.
I'd also wonder if he's simply starting to realize what most people realize at some point - usually later, but maybe he's precocious - that falling asleep is something that happens without your agency. My children used to argue with me when I'd tell them they'd fallen asleep - because they didn't know they were asleep, because that's just how sleep works. You're not conscious of being asleep, and when you are conscious you're not asleep. He may be verbalizing that he doesn't know how it works, which shows great understanding for that age.
As to why this happens, my long suspicion has been that it's just a matter of not knowing how to "turn off" the mind. It's sort of like the "don't think of the pink elephant" problem: if your mind is very active, it can be hard to turn it off. My wife and I are dissimilar here; we both have very active minds, but she's able to turn off quite effectively while I am not.
I'm also a very sound sleeper once asleep - while I know it's a meme to say that the husband sleeps through the baby cries conveniently, I truly do not wake up to anything short of a nuclear explosion once I'm asleep, until the wake-up time that I often wake up just before the alarm (and if I'm too tired or set the alarm too early, I often won't wake up to it the first go-around).
Additionally, I can't visualize images "inside" my head (meaning, if you say "imagine a horse" to me, I don't see a picture of a horse in my head), which I feel has led to it being harder to stop thinking about outside stimuli - as I don't have anything to retreat to except darkness inside my head.