instrument lighting wiring and fuse

I'm working on a 1970 Dart with relatively new M&H wiring (which was a dream to work with). On the "instrument" fuse terminal I piggybacked an extra wire to feed the added instruments in the car so they would work with those in the stock instrument panel. There are three gauges and a tach and each has its own bulb. Every once in a while the "instr" fuse blows. It is a 3 amp fuse. All the wires feeding various lights are 18 ga like the wires fed by most of the 20 amp fuses in the system.

What am I risking by replacing the fuse with a 5 or 10 amp fuse to cover the additional lights and perhaps solve the blown fuse issue?