You'd need an accumulator, pumping the mains at a rate above 12 litres per minute is illegal. 3 rain head showers at a time? Without having seen the spec of your rain head, I'd be allowing 25l/min each. That's 75l/min draw for three, and up to 100 l/min if you've got all four running at the same time. That's before you even start to take account of any appliances in the kitchen, like the dishwasher, running at the same time. You'll never get 75l/min out of your main. If you get 30l/min I'd be surprised.