As the others have said best to get rid of the third nap. We also recently started having probs with our 3rd nap but she was waking from her 2nd too early so would start to get tired at tea time, in the end I had to extend the time she was awake for between 1st and 2nd nap.
See how it goes, it could just be a phase he is going thru and might start sleeping thru again.