How you getting on? Yeah, I also have a three year old who's pretty much dropped his naps. I've given up trying most the time as he just won't go to sleep. Try and do a bit of quiet time in the afternoon with him as at least the 4 year old is at school!
Believe me, I don't like any wake up before 7am! However, we're going through the 2-1 switch right now with ds2 so he's having to go to bed at 6pm a lot of nights if he's refusing the second nap so is up usually by 630am but he's had over a 12 hour night by then so can't really expect much more.
45 min naps can be UT so maybe stretch the A time a little bit before the nap to see if that helps?