Provided your new ballast has (2)red, (2) blue (2) yellow, (1) black (1) white, and has 32W T8 lamps:
You'll have to do some rewiring to get the 4/3 lamp ballast to work with a fixture originally designed with a 3 lamp ballast.
One of the lampholders on the "red" end of the original fixture will need to be disconnected.
One of your yellow leads will go to this lampholder, the other yellow will connect to the remaining 2 lampholders.
Connect 1 blue lead from the ballast to the lampholder opposite the lampholder that's wired (to the yellow) by itself. Cap the other blue lead from the ballast.
Connect the remaining 2 lampholders, each to one of the red leads from the ballast.
Watch for smoke