what creates more heat a triple xp-g2 running in parallel or a single xp-g2 running at the same current? Also, assuming a proper heat sink, will the LEDs on a triple run cooler given there is three times the surface area for thermal transfer?
In the triple each led will be running at lower current. Efficiency goes down as current goes up.
So to directly answer your question, the single Xp-g2 will create more heat at the same current.
Let's look at some test results.
A direct bonded copper XP-G2 at 5.8 amps 875 lumens. This emitter is already near the peak output possible with a copper mcpcb and passive cooling. Lots of energy is turning into heat instead of light.
At 1.9 amps, a single XP-G2 puts out 460 lumens. Put three of those together, and it's so much more efficient that it puts out nearly 1500 lumens. Nearly twice as much energy is being converted into light instead of heat.
Not sure if this is of any interest to the OP, but the reason people often choose a single over a multi-emitter light is for throw. (Perhaps price and availability too - there a lot more singles made.)
A triple will ultimately put out more light and less heat, as well as giving better runtimes (at a given lumen output) due to the higher net efficiency.
On the other hand, a single emitter is easier to focus into a narrow beam for better throw.
Usually, single emitter lights will outperform triples for throw, while a triple will give more light but in a floody pattern. Every true flashaholic will undoubtedly have multiple examples of both kinds of lights! 
Of course, if you start getting into stupidly large triples (BTU shocker, etc), then you can have both flood and throw. 