I think you're fine for a few years yet. But as a parent and step parent to teens I think there are a few issues that become more difficult as they get older. Me and my ex never quite did 50-50 but he used to have them 2 nights a week, whilst DSC have always spent every weekend with us, but now two of them spend half the weeks as well as this. This hasn't been entirely smooth.
Once they start secondary school they have a lot more stuff to carry around and going to and from school from different houses becomes tricky. (This can happen sooner if they play instruments or sports that require equipment carried.)My DD got quite stressed about this and flat out refused to stay at her dad's on a school night a few weeks into Y7. Fortunately my ex was flexible about this and she now comes back around 8pm on a Sunday evening and just goes for tea the alternate Mondays instead of staying overnight as she used to.
They also get more fussy about clothes around that age, so having clothes in each house that just kind of cycle around doesn't work so well and both my DC have started taking clothes with them to their dad's and bringing them home again.
But the biggest issue is I think the least tangible and that is that parenting becomes less "transferable" as they get older. It's more built upon the relationship you have with them and there are more long term things to sort out, rather than day to day needs to meet. There's much less supervising of them they way you do with a 3 year old. But instead, there's chats you need to have about school subjects, things they're worried about, their health, their social lives, revision, applying for college/driving licence/passports/etc. And this sort of parenting is much harder to do if there isn't one parent clearly in charge. With my own DC it has always been me that is clearly in charge of these sorts of things, and this works. But my DSC who have previously spent around 50% of their non-school time with us, and now spend more, I think things are quite often falling between the cracks with neither parent really having an real sense of what's going on in their lives.
But that said, your DS is only 3 so sticking with 50-50 for the next 8 years or so will mean he should have a really strong relationship with both parents, and that won't go away even if you do find you need to flex it some way in the future. A system where one of you always has Monday and Tuesday nights, and the other does Wed-Thu nights, and then alternate the weekends works well to allow each parent some autonomy over clubs etc that the DC does (ie if Monday is your night, you can decide whether to let DC do cubs that night or not, and you'll be the one who gets involved in it, and your ex can do likewise with things on his nights). That system also works well to let you both work some days with no stress about childcare at the end of the day. I've a few friends have done this and it's worked well with primary school aged kids.