2 shower heads and 1 rain head...low flow
You have 3 shower heads on 1, 1/2" supply line The demand is higher than the 1/2" can supply. The size of the valves also makes a difference with high flow shower heads. Both the supply line and possibly the valve will need to be increased in size for full pressure with all 3 heads turned on. A booster pump or increase in pressure may be another option.