Think i get you. The lights dimming would be as a result of the cabling being too small. Nowt to do with the fuse. Fuse just protects the cable from overloads. Switch rating should be adequate.
Fuses are to protect the wiring. So putting the fuses as far back towards the battery on each wire is best. Sharing a fuse between the LEDs and a pump will almost certainly pop the 5A fuse every time you turn on the pump. Because when a DC motor is standing still it is only the DC resistance of the rotor windings across the battery. A "3 amp" motor may well draw 15 amps until it starts moving then the current backs off. The fuse should be sized to blow if the pump stalls and not blow if it spins up in time. Expect to blow a few fuses until you get the right size starting at 5 amps.
