The 3 year old boy could be peeing on you or peeing on himself when you open the diaper because he gets enjoyment out of peeing on you.
My 3 year old boy did that same thing he would pee on me when I opened up his diaper so he was purposely holding his pee in and was peeing on me as soon as I opened his old diaper.
However if he started doing that I would place the old diaper back on him as soon as he started peeing.
The old diaper was still under his butt and so I would just put the old diaper back over his penis until he stopped peeing and that stopped me from being peed on.
My son thought it was funny and at times I did too but I did spank him late for doing that and eventually he stopped.
So just don't fully remove his diaper and just pull the diaper away from his penis and allow hims to start peeing and then immediately put the diaper back over his penis to catch the pee.
It could also be air hitting his penis which can stimulate the penis and start the flow of pee.