I don't know Euro wiring but, in the USA, if it has just 3 wires, it will be single phase (1 phase).
The thing you need to know, is if it's 110v or 220v (basic terms). I don't know what "0" means in Euro wiring but, we have something called a "common" for 110. It's basically another ground.
Use a Ohmmeter and test the wires to the welders case. If 2 wires indicate a ground, it will be 110v. If only one (the green stripe maybe?) is grounded, it should be 220v.