Do you have a multimeter to check resistance/conductivity?
If there are 3 terminals it's a SPDT switch. To use as a basic SPST on/off switch you only need to use two terminals.
The center is common and the other terminals are usually physically-opposite of the switch position... If the toggle is up, the center and bottom terminals are usually connected.
Since it's a center-off switch you'll have two off positions. (Or you can wire the other two terminals together so it's only off in the center position.)
DPDT switches have 6 terminals and they are basically 2 SPDT switches that work together from the same toggle. Again, you can use it as an SPST switch if you only use 2 terminals.
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