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You need a phase converter to convert the single phase to 3 phases, but if you dont have enough current to run a single phase, you wont be able to run a bigger 3 phase welder.

What sort of plug does it use? How many amp? What size cables is the shed wired with, will need to be 16mm cables to handle that current. Unless you extension cables are also 16mm then they are too...

Anything over 160 amps will struggle on a single phase line, unless the building is wired for it.

Your 260 amp welder is probably lacking input current, upgrading to a bigger single phase welder wont help that, you will need to re wire your building with probably 16mm cables to feed the welder.