The indicator is part of the total resistance in the circuit - thus it changes the voltage and current along WITH the two other glow plugs. Most indicators are simply a wound wire that gets hot and glows along with the glowing of the glow plugs, by bypassing the indicator - you are simply placing more current to the remaining plugs. YOU could bypass ONE of the glow plugs and that ONE would get hot in less than half the time!
The glow plug indicator is a simple design to let the human "know" when the plugs were hot!
Now days that is not the case - a circuit is constantly measuring the total resistance as the plugs heat up (once they start to heat - their resistance go up) and once it reaches a desired resistance - a idiot light comes on or off and tells the operator that the plugs are ready.
If you are the only operator - you could simple bypass the indicator and install a "momentary on" switch - or better yet a relay that can handle that total amperage, and count to "20" and let go!