Hi,
No it seems like TMR1 and TMR3 has it's external clock input connected to the same physical PIC pin (Pin15 on the 4550). That means that you can't use both TMR1 and TMR3 as counters with external clockinput. You would still be able to use, for example, TMR1 as a counter and TMR3 as timer (internaly clocked) though.

So TMR0 and TMR3 as counters should be fine!

/Henrik Olsson.