There are two types of three terminal flashers. One will work: it has ground, power, and load terminals. The other type will not work, there's no ground terminal and it just blows fuses since whatever terminal you attach the green wire to will ave power ... zzzzt!
These flashers seem to all have DIN terminal numbers - the code is a bit weird but I think the one you want should have these numbers on the terminals: 31 or 31b for ground, 51, 81, or 84 for power, and 49 or 49a for the output.
There are a bunch of posts here about this. Search for "flasher" or I think there are some example part numbers for "grote" or "tridon".