Run in 2-D and jump in a "third!".
Thanks, Nemezes! I also have a similar one that works better, without that bug you mentioned:
scirra.com/arcade/tutorial-games/on-summer-nights-5508
If you want to edit the capx to that game, you'll need to make the change I talk about in its game description.
The math works the same as with the platform behavior, just in 3-d. There's a positive, negative, or zero acceleration, then speed is itself plus the acceleration times dt, then position is current position plus speed times dt. It's basically that process 3 times.