yeah, 5 mos is right around when most babies start to do another sleep transition. usually at that age, they're either taking 3 naps but still not going to bed for the night until closer to 9pm, or they're down to two naps, but down for the night by like 6:30. if you want to drop the nap, go for it, but anticipate needing to put her down earlier than 7 or 7:30; she'll be a disaster by then, and you'll start having to deal with sleep issues stemming from being over-tired. or, you can continue the third nap, but anticipate bedtime being pushed back. if she can do either, then you're in the lucky position of being able to choose what works for you, for now, but soon she will start really needing to drop the nap and adopt the earlier bed time.
good luck! the first year is a crazy ride in terms of sleep....