Need help with purchaseing a new 3rd member

It really doesn't matter. As long as it's a 9" and has the same spline as your axles, it'll work.

the bearing size refers to the axle tubes. the small bearings hae a step down like the 8" axle tubes, called 9" small bearing. the large bearing refers to the straight tubes that stay "large" all the way to the flange. the third member is not effected. however, a 9.5 wont work with a 9" housing, not sure tho if a 9+ will.