My best guess is your secondary has two 120 volt coils. You connect them in parallel to get 120 volt only. You connect them in series and ground the mid point to get 120/240. Connect your 120 volt loads to either line and the mid point (neutral). Connect your 240 volt loads to the two lines (outer points).
If you can't figure this out get some help. This is about the simplest transformer there is to connect and is exactly what most single phase supplies consist of outside of a two wire only secondary.
You also need to understand how to ground the secondary - no matter which way it is connected.
If secondary only has three leads then the output is 120/240.
