What size line is feeding the house now and what material is it? If you have a PRV I would increase the pressure as high as you can get it. With 3 heads running at the same time you will need as many # pressure as you can get and the small piece of copper may affect it.
What size is your water heater. At 7.5 GPM your hot water won't last long...maybe 10 min. or less depending on time of year and the temp setting of the water heater.