My daugther had similar difficulties at that age as she slowly gave up her nap (she's 4.5 now and naps one weekend day typically). Her bedtime varied too and seemed somewhat dependent on if there was a nap and how long it was, but not always.
I finally settled into an 8:30pm bedtime (I need the rest of the evening to prepare for the next day) and let her turn on a small light and read if she wanted. Sometimes she would read until 10pm, other times she would go to sleep quickly. Once she stopped napping during the week, I moved bedtime closer to 8pm and she usually goes right to sleep.
I think you're just going through a transition period and may have the variability until the naps get dropped completely. Sounds normal!