it's hard to be patient after 3 years of diapers! (and 2 years of TWO in diapers, i'm sure!) my son is 22 months and we recently bought the potty ring for the toilet...he went two mornings in a row, then NOTHING, even refusing to sit on it a day or two this past week. frustrating! BUT i haven't pushed it because he is only 22 months, and because he's a boy and i've heard they take longer (your situation, case in point!), AND because pushing, in my mind, will only make it harder for him. so we've gone several days without even talking about the potty. (except i hear him jabber about it pretty frequently, and i ask him if he has to go and he says, "NO!")
i'm sorry i don't have any real advice...just wanted share your pain. although, you can look at it like i do...sometimes it's a lot easier to just plop down and change a diaper, than to take off the shoes, take off the pants, take off the diaper/pullup, sit with them for five minutes, then put it all back on...at least...i try to tell myself this is easier lol. it doesn't always work...:)