CFS3 is engineered differently than CFS(1) and CFS2, CFS2 sharing the basic, howbeit, updated format of CFS1. Of the three, of course, CFS1 requires the least amount of drive space but, at this late date, CFS2 has more avaliable add-ons (although many add-ons labelled CFS2 are actually CFS1 compatible). CFS2 also has a visual sizing difference that bothers me but would probably be of no consequence to others; most CFS2 graphics are improved except for those directly borrowed from CFS1 (unanimated hexagonal wheels are not uncommon in CFS1). Gun shells and engine smoke are common fsx for CFS2, not in CFS1.

CFS3, being a different program engine, is almost alien to CFS1 and CFS2. If you're into personal file modifications, personalizing aircraft textures, etc., CFS1 or 2 would be (ie, are) my choices; my personal preference being CFS1 since it's an easy install and also easier to correct my modification mistakes.