I would still check regularly - there are very few people that I trust to get it right. In the long term, it's not an issue - but there's a chance of unexpected pregnancy and fights breaking out until they are all neutered and hormone levels have settled down. Boys are fertile for up to 6 weeks after neutering, which gives a slight risk of early pregnancy for the girls if they are all done as early as your vet advises.
If they do start to fall out, they will need to be temporarily separated and rebonded after neutering. Don't leave them if it looks like a serious fight is brewing (locking on to each other & drawing blood, rather than a bit of a chase and fur pulling) or one is being intimidated too much as they may never get on again and you will have to rethink the trio.
Meanwhile, enjoy them. Trios are so much more fun.