There are a number of electronic ballasts that will operate:
2 lamp ballasts (2 blue leads, 1 red) that will operate 1 lamp, just by capping off 1 of the blue leads.
3 lamp ballasts (3 blue leads, 1 red) that will operate 2 lamps, just by capping off 1 of the blue leads.
4 lamp ballasts (2 red, 2 blue, 2 yellow) that will operate 3 lamps with 1 of either the red or blue leads capped.
To retrofit a 4 lamp ballast into a 3 lamp fixture (originally equipped with a 3 blue, 1 red ballast), cap 1 of the red or blue leads.
Remove 1 of the jumpers from the lampholder to lampholder on the common side (originally red). The 2 yellows will go to the lampholders on the common end, 1 to the (originally red) lampholders, and 1 to the lampholder freed up by removing the jumper.
.........or just buy a 3 lamp ballast
Just like the black and white wires that will operate on 120 to 277 volts, they are meant to reduce the number of different ballasts you have to lug around on a day to day basis.