I think the company may have tax obligations regardless, so therefore they still may wish that you set up as a limited company, or else they will have to fill in a load of internal tax compliance forms. Lots of admin for them that they may try to avoid at all costs!
My recommendation is that you push the sole trader issue with them and tell them it's not in your interest to set up as a company. See how far you can push them. If they insist on the company route, and if it's only for three months, then it might be worth looking at the umbrella route.
Words of warning though - I know a couple of people who started for "only three months" with the umbrella strategy and are now regretting it 12-18 months later (i.e. with hindsight if they knew it would have been for longer they would have set up a company).