Overload Wiring

A 3Ph O/L can be used for single phase if connected as shown

Hmmm... this is actually more complex that it seems on the surface, and I may get typing cramps before I'm done. It depends on where you are, or actually where your components were made.

1. If you are in any country OUTSIDE of North America, then there is a 99.9999% chance you will have to use the wiring scheme shown by TonyS above for a single phase motor. This is because most IEC overload relays have a feature that will, in effect, alter the trip curve lower if power is not flowing through all 3 pole of the device. The purpose is to protect 3 phase motors from single phasing because if you lose a phase while the motor is running, it will heat up FASTER at the same amount of current draw that if it had all 3 phases. But I digress... If you have a single phase motor then, an IEC OL relay is wired as Tony showed so that current is passing through all 3 legs. It doesn't matter that one of those circuits is already passing through the other leg, so long as there is current moving through all 3.
2. If you are in North America, AND you have an old NEMA style bi-metal or eutectic melting alloy OL relay, then that phase loss sensitivity is NOT present and you do NOT need to have power flowing through all 3 legs. So for a single phase motor, where one of the legs is NOT the neutral (i.e. 240V), then yes, you would only run through 2 of the 3 poles. On an old NEMA starter it will just ignore the fact that there is no current on the 3rd pole.
3. If you are in North America, AND you have a new IEC style OL relay, as is the case with many mfrs now, then the same issues in item 1 apply.
4. If you are in North America, AND you have a MANUAL motor starter of the old NEMA style, and you have a 120V (Line to Neutral) single phase circuit, then you CANNOT run the Neutral through the OL device, because Manual Motor Starters switch the power poles, not just the coil circuit, and you are not permitted to break the grounded Neutral of a circuit to a motor.
5. If you are in North America, AND you have a MANUAL motor starter of the IEC style, then the issues of item 1 apply, except again, you cannot break the Neutral with the switch, so you must loop the one Hot wire through ALL THREE of the poles of the device.

Whew...

Side issue: Someone mentioned having seen only 2 OL heaters on 3 phase starters years ago. That WAS true, but became ILLEGAL in 1978 with a change in the NEC that now requires that there is an overload protection device in ALL ungrounded conductors of a motor circuit, single or 3 phase. So if you see one now on a 3 phase motor, it is either older than 1978 and grandfathered in, or you have found one that someone bought off of Fleabay and installed without an inspection, because an Electrical Inspector would have nailed that as a Code violation.