Three way valve bathtub/shower problems.?

My bathtub/shower’s hot water will not turn on. I have a three-way valve system and have taken off all the knobs and I’m now looking at 3 valves on the other side of the tile. Could there be something else that would prevent my hot water to turn on other then a bad valve. My house is on a slab and I don’t know if that matters. Bonus for me is there is no access panel to get to these valves. I’m going to have to go into the wall. Before I do this I’d like to trouble shoot any other reasons why my hot water wont run when I open the valve. I mentioned that my house is on a slab so I don’t know if that matters. Nor do I know if the water lines run through the concrete like the sewer pipes do. And kind of enlightenment or advice anyone might have would be very welcomed. I don’t want to have to go into my wall to fix this plumping. If I do I might as well gut and update everything. Help me not create a big project I’m not ready to do yet.