I'd skip the triple. Most triathlons do not have the kind of climbs that need very low gearing. As for the disc, I'd save my money for a triathlon specific bike rather than buy a disc. (as a second bike, I think your plan of buying a road bike makes sense)
A rear disc is going to make less difference in performance than rider position. Also the disc is only going to be useful in races without significant climbing, and when the wind isn't heavy. And some races (such as Kona) don't allow discs. If you're spending money on aero wheels, the front is more important anyway. For wheels, I'd invest in a set of deep dish wheels (Hed Alps, or Zipps) before I bought a disc. A lot more generally useful.
My 2 cents, buy a decent road bike and clip ons. Ride it for awhile, do some events, and then decide on your next purchase