You report a very strange situation - in your position I would call in a good and properly qualified electrician to inspect the house wiring.
I suspect that the original installation was by a plumber and that he made a mistake with the wiring - I can't imagine what or how.
As for too much or too little power the shower (any appliance) will draw the power it needs - if your shower is rated at 3kW (3000 Watts) it will draw that amount of power providing the voltage is in the 230V range - which it will be. Similarly if it's a 7kW shower.
Your report of the house lights dimming is very worrying - if the lights dim when you use the sower it can only be a voltage drop - and that shouldn't happen - a shower on its own circuit should not cause a voltage drop on a lighting circuit - the electricity company supply will be able to cope with all your appliance working at the same time.
You have a problem with the house wiring.
I also have some doubts about the three failing showers and the two failing switches.
Have it inspected...
December 2015