I am in the middle of a bath remodel. The original copper lines in the wall for the shower had the 1/2" supply for hot capped, probably due to a leak or low pressure. It was done when the house was built. They put a T on the garden tub stub up which is in the middle of the floor and ran that line under the shower pan to the shower wet wall next to the cold supply. When I tore out the tub and shower I discovered that I have no way to supply the new shower. The water heater is on the other side of the house, and this remodel bath has no attic.
I don't want to tear up the slab if possible.
Can I run 1/2" pex off a 1/2" copper supply under the sink through the wall and still get enough water to supply a 3/4" valve and 3 separate volume controls feeding 3 shower heads? Will I lose volume through the pex connection, or the 15' of pex I need to do the run?

Thanks