I have no interest in tris but... welcome to the forum!
I would say - if you're going to to sprint length tris, you can probably get away with anything, including a normal road bike with clip on aero bars. If you're going to be doing seriously long time trials then you would probably benefit form an actual TT bike.
if you can only have one bike, then lean towards road bike with bars you can clip on or off at will. "comfort geometry" road bikes tend to have a higher head tube, which can be counter productive for getting into an aero position, but they're dead handy for older, fatter guys like us who want to ride a bunch.