I've only done this with a 2x key over two switches and it is very solid, but the point of stabs is that when you press one end the other end is pushed down too. If you pressed over the end switch it would not be connected to the other end so it wouldn't press straight dow. I also think you'd need springs in them else they might rattle, but all thumbs are strong enough to take 3x light springs so shouldn't be a problem.
The best thing to do is make a "plate" out of some thick card or cardboard and see what happens

I also think you'd need springs in the end switches else they might rattle, but thumbs are strong enough to press 3x light springs so that shouldn't be a problem.