run the 3/4" pipe all the way to the shower. Branch off for the lav and toilet with 1/2". Your pressure starts with what is supplied at the street. The smaller the pipe the lower the volume that can flow through it and thusly the lower the pressure. Last thing you want is to get scalded in the shower when someone flushes the toilet so don't do anything to restrict the flow any further. Contact a plumber asap. Post this over at www.plbg.com and they will explain the volume and pressure dynamics of plumbing.