I'm sure you'll hear from folks with much better credentials than I, including some real marine mechanics here, but I wouldnt do it.
I think a 35 lb TM can pull over 20 amps and 12 gauge wont do that. If you fuse ( or circuit breaker) the 12 gauge at a low enough amperage to make sure you dont fry it, you wont be putting getting enough power to your TM to run it properly, that if you are running a locator and a TM on the same wire, you may lose the reading on your locator every time the TM is on or blow the internal fuse in the locator every time you hit the TM, and that by doing all this plus the electric start outboard on one battery, if your TM runs the battery down too low, you could find that you can't start your outboard.
I'd get two batteries, one for the electric start outboard and the locator, and the other just for the TM. I'd put the starting battery somewhere in the back, run 6 gauge from it to the outboard, and then run 16 or 18 g ( memory fades) from the cranking battery to the locator, with a 3 amp fuse close to the battery. 3 amp will run an older locator just fine, the newer mapping ones may pull more amps, I'm not sure. I ran older locators including graphs wired that way for years without problems.
Then I'd put the troller battery however far forward I needed for weight distribution, and run just the TM on it with 6 gauge. The extra cost this way would be the second battery and maybe $30 more for heavier wiring. If you were willing to risk the dead battery when you wanted to start the outboard, you could run all three off one battery, with three sets of wiring and see if you could get by without battery #2.
If any of the mechanics chime in, please go by their recommendations, not mine.