Warren,
It's quite possible to use all three of the major current mods, plus Multi-player, on the same machine. However, you cannot , without fiddling around (the exception is - you can run WFP on top of FCJ - you get all the WFP a/c with FCJ underpinnings) run one of the major patches on top of one another. I, and many RBers, have multiple installs running side by side - and I fly FCJ, WFP or HA as the mood suits. As has been noted in several threads here, each major patch has it's advantages. HA is free, easy to install, and completely replaces the stock game with updated a/c graphics, historical markings, sounds etc. WFP is a date based patch, which means that it enables a/c, squadrons etc to be introduced and retired as the war progresses - there are more than 100 flyable a/c and variants in the WFP stable. It will also have a moveable front - due to be released in the next two or three weeks. However, use of this patch requires some maintenance. FCJ has first and foremost, superb photorealistic graphics, but it also has much more underlying it. Again, you can overlay FCJ with WFP, but you lose the photorealistic FCJ a/c skins.
In addition, you can mix and match minor patches - principally terrain, Flight and Damage Models, with all three, though each comes with it's own recommended set. For technical help, see the three major Fora - we all hang out there, and someone much cleverer than I is always eager to help...
http://ac.prospero.com/swwisa/start
http://ac.prospero.com/TheWesternFront/start
http://ac.prospero.com/thepromisedland/start
hope this helps,
shredward