right I haven't read through every reply but I have had 5 children including 3 boys. First of all ditch the potties boys work better standing up. let them watch you go. Also modern nappies are so absorbant that children don't know they have wet themselves really, so, messy though it is, let them run around naked sometimes or with pants on so that they do have accidents. do this for a week if you dare. They will get the hang of it. Do not show any anger when clearing up. just change them and get on with something else. At 3 1/2 they could also have a award chart with a special treat when they complete a week.
Ask the health visitor for help and other mums for their ideas and remember no healthy normal child goes to school in nappies.
And buy a toilet seat for them.
Good luck!