The lampholders on the left side of lamp 2 and 3 need to be disconnected from each other and rewired as individual wires for the new parallel ballast as shown above. The lampholder on the right side of lamp 3 will be wired from Individual to common and connected to the lamp 1 and 2 right side common wires.
In the left image, the wire is wrapped around the fingers of the dominant hand as shown. In the right image the dominant hand is closed, while grasping the lampholder in the other hand. The wire is gently pulled while twisting the dominant hand until the wire comes out.
A 3 lamp series-
There are three options for replacing a 3 lamp series-
Ballast with Parallel