Any mild acid will loosen or remove rust from steel. Problem is, if you use a strong acid, it will etch the steel, too, which you probably don't want. Phosphoric acid is often used because it reacts with the underlying steel much more slowly than with the rust. It coats the steel (mostly, barring a few too many pinholes) with iron phosphate, which you really don't want if the steel will be electroplated. It's fine if you plan to paint it.
I use hydrochloric acid (sold as muriatic acid) about a 25% solution, for rust removal and as a pre-plating "pickle." It won't dissolve the rust, at least not in a reasonable time, but will loosen it quickly so you can brush it off easily with a hand-held wire brush.
Do you have a copy of Roger Williams' book on TR restoration? It contains a lot of info specifically about TR3 door reskinning.