Sounds like your daughter is transitioning from 1 nap a day to 0 naps a day. And lemme tell ya, I despise that transition because it *is* tough - they do seem tired, but then the battle is barely worth it AND they're staying up later than their usual bedtime. This is how you know that it's likely a true transitional time - if she were tired and then went to bed at her normal time, I'd fight for the nap. But struggling so hard for the nap and *then* having to struggle with a later bedtime - when we were in this exact same position with my daughter, we did the math and found out that on days that she napped, she ended up getting overall FEWER hours of sleep due to the later bedtime.
I know every child and family is different, so don't know if this will apply to you & yours, but here's what we did:
1) "Quiet time" - reassure your daughter that she does not have to sleep, and describe (short & sweet) to your daughter that she'll have quiet time instead when she looks tired, help her recognize these signs - for example, "Honey, I see you rubbing your eyes and yawning which shows that you're a little tired. You don't have to nap or sleep! However, big girls like you do need to have some quiet time in your room for 30 minutes..." - lengthen this to 45 minutes, then an hour, and as others have suggested, provide lots of quiet things for her to do on her bed (books, stuffed animals, finger puppets, etc.).
2) ***EARLIER BEDTIME*** When they give up their naps, it's really helpful to bump the bedtime to an earlier time by at least 30 minutes to 1 hr - or even earlier if you're watching them and they show signs of being tired/sleepy. That might mean a 6:30pm bedtime for a month or 2 until that transition from 1 nap to none is complete.
Best of luck - I see that you have a little baby boy, too, which parallels our situation too! My daughter is 35 months older than my son, and it was incredibly brutal for her to drop that last, glorious afternoon nap before the baby came. For a while I thought I was going to lose my mind trying to juggle a non-napping 3 yr old and a newborn, but it does get better. :) Hang in there... this, too, shall pass!