Hi,
I bought a wellpump, but by accident I got a 240v 3 wire, it cannot be connected to a 240 v circuit 2-wire?

Are the 3 wires 3 phase or 2 hots and a ground?

Is the wiring on the new pump, Black, Red or White with a bare or green ground?

If it is, then you have all that is necessary for a 240V connection. Pure 240 V does not use a Neutral (White). That's why a white that has black electrical tape wrapped around can be used. The black tape signals that this white conductor is being used as a "Hot" conductor.

However, you must make sure that you use the correct size conductors for this motor.

No, 3-Wire models require an above-ground control box between pressure switch and pump motor.