I'd have to agree that Mig will do most jobs but there are occasions when TIG and ARC are more suitable. If you have a workshop and 3-phase, then I'm assuming you're setting up something a bit more professional than a garden shed workshop? If so, get a 3-phase MIG welder and a small inverter TIG/ARC. The latter will often be combined into one set as they're almost identical electrically. Inverter driven arc welders are an absolute joy to use. I bought one of these last year for £300. It's been better than expected, however it drinks Argon faster than I can drink scrumpy!

SIP (S.I.P) 05266 Weldmate Welder P178 (160amp) HF Tig/MMA/Invert Welding | eBay