Generally, the two opposing or outside pins are connected together; Check them for zero resistance. The usual reason for 3 pins is for mounting stability, so you don't move the variable cap when you adjust it.
Most capacitors are just two pins with the third pin connected to one of the others. The exception to this are butterfly and differential capacitors, which are dual capacitors with a common pin, but usually these are larger air variable capacitors and I am assuming from your drawing that yours is instead a relatively small (10mm diameter or smaller) trimmer capacitor.
