I agree with the last suggestion- it may be time to modify your schedule. Mine dropped her third nap at that age, too. Try two naps, and once she is adjusted she may sleep a little longer those two times, or she may sleep longer at night (at that age mine was going to bed at 7, which my MIL thought was awful, but it was best for Izzy). I also think she's too old to start swaddling- if you had already been doing it, it might be okay, but she will fight you like crazy, and if she did get used to it it would be a harder habit to break, plus babies that age don't need to be swaddled for the same reasons younger infants do.
I don't know if it would help, but look at her eatting schedule, too. And remember, right before babies grow they almost always sleep less and in general act funny (for lack of a better word- mine will give me an almost sleepless night and not eat hardly at all right before she grows, but it's almost always just a one or two night thing and then she's over it).