Hi!
Around this age I found my DD's 3rd A time needed to be longer than the first and second A times...she always seemed to increase her mid-late afternoon A times before her morning ones.
Have you tried pushing just that 3rd A time to 1.5hrs, I.e. Starting 3rd nap at 4pm and see if you get a longer nap?
I suppose that would clash with school run though, so that 3rd nap might need to be in car/buggy on those days.