Thanks for the additional comments. At this point, all I want is to understand how to write two ISR for a PIC18F27K42 in which upon an overflow interrupt from Timer3 or Timer1, the appropriate ISR increments a counter, clears the interrupt flag, and returns back to the executing PBP subroutine. The PBP routine uses one overflow count to compute a time period . The other overflow count is used to timeout the subroutine . The application is a tachometer that measures RPM from 0 to 12K. My code works with a PIC18F26K22 and DT_INTS-18. I have tried polling the interrupt flags within the PBP subroutine, but at low RPMs and clocking the PIC at 64MHz the results are erratic.

Yes, I could just keep using an 18F26K22, but why not learn something new?