Thats a great question, and Honestly I dont know. Though I do have brushed experiance, and know how they operate and how they are built, the actual speed difference is difficult to actually answer in MPH.
I have been running BL motors for around 3 years, so its been a little while.
If I remember correctly, your final RPM will be equilivant, so your top end speed - on a perfectly flat road should be the same.
You will gain speed depending on the terrain your on. If your on street with high traction with and a lot of turns, the 12 single can give you faster lap times, because you can blast full throttle, using the torque to get you out of all of the corners.
If your traction is not perfect, the 12 tripple can be faster as it will not break the tires loose as easily, giving you less tire spin and more go.
Though its a rough explanation - I hope it helps.
If your just playing around, and not racing for lap times, you will more than likely not notice a real difference.
Original RC 10T, T4, RC8Te, Tekin RX8. Losi Ten SCTE, Slash 4x4.