So the jugs are loose but the pistons are stuck?
Soak with rust buster, block up the cylinders as high as they will go above the crank on fitted wood blocking. Cut a chunk of 2x4 to fit the bore and piston crown start beating, all three jugs in turn, round and round and pound them down. But My 2 cents? Throw it out and find a better engine. I doubt it is economically feasible to bore out and put new pistons and rings in an XS750.........
Unlike an XS650 with an XS750 some pistons are going up while others go down so you are going to have to keep raising the cylinder higher above the cases.