I was thinking about buying a 3PL which comes with a 4-channel receiver, and I also wanted to get more receivers, but I was wondering if the receivers that come with the 2PL (2.4GHz Version) would aslo work with the 3PL.

Appears that at least one rcvr, R2004GF, will work on both 3PL and 2PL 2.4Ghz systems. That rcvr is listed for both radios and its also the less expensive. The R603GF may work as well, but its more expensive since its 4ch.