In X3 each shield requires a certain amount of energy to recharge at the maximum speed. A 25MJ shield requires 250kW to recharge in 1min, 51sec (the maximum speed).
If a ship has three shields, it will require 750kW (3x 250kW) to recharge the shields at the maximum speed. If a ship has only half the energy (375kW),
then the shields will recharge at half the speed (so 3mins, 42secs). If the ship has more reactor energy than required, it will not make the shields any quicker than the maximum speed.
All shields share energy equally.