his pm nap seems very late in the day so his day is rather long, 14 hours.
i would probably try bringing his morning nap earlier, so that's the shorter one, then bring the pm nap earlier so that he's up from that around 4 so ready for bed around 8. so you could try his am nap at say 10, for 45min, and then put him down for his pm nap at 2.
it depends on your babies sleep needs, my DS isn't high sleep needs as such but he wasnt ready for 1 nap till around 18mths but prior to that we had about 3 months of a schedule similar to what I wrote above-which went wonky eventually so we ended up on 1 nap. My instinct is that 12mths is a bit early for a full on 1 nap, unless your LO is very slow sleep needs. There are plenty of LO's here who have done that, but if your LO isn't low sleep need, going to 1 nap might just create more OT.
3hr day sleep seems about right.