No, long am and short pm is fine, that's what many people do - you push the morning nap later and make the pm nap shorter and shorter until you drop it completely. BUT, if your LO tends to refuse the pm nap, it can be better to go the short am route. It can take a few days for your LO to get used to it, so you may end up with several days of two short naps - but eventually as you get the timings right and her body clock gets used to it, you should end up with a nice long pm nap.
I really think if your LO is only managing 3.5hr A in the morning you'd be better off keeping two naps for a while (as far as nursery allows). So perhaps something like this:
WU: 6.30
nap: 10 - 10.45
nap: 1.45 - 3.15/30 (but don't cap - let her sleep)
BT: 7.30
What do you think? If she won't go down for the pm nap until later, or if she wants to do such a long pm nap that bt gets too late, you can cut the am nap to 30 mins and so on.
Posted with oneplusone
At this stage things can change a lot in 2-4 weeks so I would go with the two naps now and she might be more ready to cope with one nap by then.