if you just sitch 36 volts through two wheel chair motors, (that are 12 volts) several bad things will happen.
first, you'll break the motors, they just wern't designed to run at that sort of voltage, and the current drawn over the 3 batteries will be really quite ridiculous.
second, assuming the motors do survive, (which I doubt) they will be spinning far too fast, you 'd still need to sort out a kind of throttle control. controlling the voltages supplied to the battery to alter the speed to the sheels.
and, if you want my opinion on that, it's not going to be easy, you're going to have to use some serious electrical switches to handle the currents involved.