At 3yrs old and just been cut i think you need to look at him like he was a colt... as it does take a little while for the hormones to disappear and maybe trying to take him away from the mare would help.
To me its sounds like a domance(sp) issue... as you say he's bossing the other horses about and now trying to boss you about... firm but fair handling and setting the rules now will help.
Getting one his own size may.. or may not help... since he's already got company so its not a lack of company.
I've got miniatures and they never behave like this so its not a breed trait... what was he like when you first got him? as some minis are allowed to get away with murder because they are little and cute..... and this is where a lot of these problem come from.
Depending where you are i'd be willing to see if i could come out and help you... can't be sure of anything but i do have the experiance in the little ones so i can help i will.
RIP Halo - 1998 - 2006. Miss you always.
My little
horrors darlings.