If you're setting an exam that has around 1hr15m of content, and allocate 1hr30m, but allow candidates to remain for up to 3 hours.
A whole lot of candidates will stay right up until the end - getting the equivalent of 100% extra time. So they will have an actual advantage.
You will get some candidates thinking 3 hours is the length, and they will think they should get t 25% extra on top of that.
You'll have longer exam periods as there's more clashes.
Candidates with rest breaks could easily be in the room for 4-5+ hours, after all, they're allowed a maximum of 3 hours. Or does this 3 hours include their rest breaks?
And just because your child would have liked 20 minutes, doesn't mean he needed it, he probably didn't manage his time well, or was overly verbose in his answers anyway.