As things stood now you have two option
(i) Use latest processor having more then 3 timer/counter plus PCA.

(ii) But as I have said earlier my scheme will work perfectly with slow count rate upto 500 - 1000 pulses/sec .(may be higer with higher speed 51 proc avai).
There is very little processor overhead soft counting during ISR. If you do 3 polling for 3 counter/timer max it will go to 25 %. Even using this schem you can implement low speed software serial comm.
just keep timer period = 1 or 1/2 clock period of RS-232 bit(for 300 BPS it will be 1000 ms/300 = 3.33 msec).
Everythinh depends upon your system design i.e what other things you are doing

Anil