Here is a sketch of the front view. These are not the exact proportions, but this would be the general idea, organized in the order that triathlons general proceed: swim, bike run. I realize that you initially suggested a biker's leg, but I thought this might be problematic because of the bike wheel. The biker's bars and helmet could be modeled after your own equipment. I could also incorporate your sneakers into the runners leg, and if you'd like, and/or put a wetsuit on the swimmer arm. Below are the profile views, with an alternative suggestion for the swim section (which would take more time)
Here is a sketch of the side view of the swim arm. I was thinking of an arm underwater, pushing through the water. But the hand could be turned and the elbow bent to convey the idea of an out of water stroke. In order to see as much of the sculpture as possible, I would mount each one on a board (of a darker wood, somewhat similar to your door and/or ceiling), where the board would be installed diagonally in the recess (this way the lights on the top "ceiling" of each recess could still be used). The wood for each piece would be basswood, which is light in color.
Biker Profile: You would see some of the face since this would be viewed from below.
Runner profile
This is an alternative idea for the swimmer, which would take a bit more time/hours