Yes, it will work. 3PK runs in one of 3 transmit modes - HRS, PCM and PPM. You select the mode via the various menus in the radio. PPM is the one compatible with the older PPM mode. The Futaba R113ip receivers use PCM and Futaba R203HS uses the HRS mode. HRS requires digitial servos and is not to be used with analog servos.
You will not have the internal low voltage or the loss of signal failsafes when running in the PPM mode - that is one thing the older rxs didn't have. Ebay is a great source for rxs - I but mine there when I need some - I run the R113ip PCM rxs so I can have the failsafes, but I have 18th rcs that use ppm (Novak XXL rx - PPM) cause there is so little room. I also run a FAAST 2.4Ghz in a few rcs as well.
There are other brand receivers that will work with the 3PK, you just have to know what mode they run on.
Make sure you get the manual or go to Futabas site and download it. Its complicated enough that it'll be a royal pia to try and figure out on your own, and it's simple once you've got the manual! Its a great radio, has a ton of features and is very flexible. I've had mine almost 4yrs and run nearly everything I own with it.