I think Tony has you covered. You probably need a 50 gal water heater. Run 3/4 inch all the way to the shower valves before reducing down. Branch off to lavatories and toilets with 1/2 inch. The extra shower head will make it about like a third shower.
That said, volume and pressure are all relative and proportional. Pressure drops over long runs. It all starts with the pressure you city provides at the meter, the size of meter, size and distance of line coming into the house and going to your water heater. The larger the meter, line and pressure coming in the better off you will be. Motels use around 2" main lines with 1" or 3/4" inch run to individual rooms with recirculator lines run back to the hot water heater which has a pump to recirculate the hot water so it will always have instant hot water.