Are you talking about connecting to the house panel?
Finding the wiring diagram would be helpful. Do you have the instruction manual?
You need to figure out of that 3d prong is the neutral (it probably is), then figure out if it is bonded to the generator frame. That is just a few minutes with your meter.
Use the neutral and the ground on the 120v outlets as references.
If this is the neutral and isolated from the frame you would connect it as a non-SDS supply. The transfer switch will transfer the ungrounded connection and the neutral is a direct connection along with an EGC to the generator frame that you will need to add.
