Hmmm. I know when my dd's naps got shorter it was time to increase her A time. Do you think you could lengthen the A time between nap1 and nap2 and then again between nap2 and nap3. You might need to work towards dropping nap3 over the next month.
I got there by increasing A time in 10mins intervals even if it meant I had to hold her for the whole 10 mins. I had to completly ignore her tired signs (ulness she was having a meltdown) and she adjusted over a few days. I bet you can get nap2 and subsequently nap 3, delayed by an hour over a weeks time.
Good Luck, Tarri