i have 3phase auto transformer, but two windings are short circuited. now i like to use 3phase as a 1phase auto transformer? how could i connect?

Problem will be that the active phase will still induce a voltage on the unconnected phase and unless the short are open circuited it wil cartu short circuit curent, albeit much less and end up heating the transformer.

Its efficiency in less.

If you need a single phase unit, get one, or fix the one you have.

Use a hacksaw to cut the shorted windings away, then you can use the remainder.