My take (but the fractions in the third element are always converted from `a/(b/c)` to `ac/b`, such is life).

    transform = 
     {Total@#1[[1 ;; -2, 1]],
      Sqrt[Total@(#1[[1 ;; -2, 2]]^2)], 
      #1[[-1, 3]]/((#1[[1 ;; -2, 1]]).(#1[[1 ;; -2, 3]])/Total@#1[[1 ;; -2, 1]])
     } & 
    
    transform[list[[1;;#]]]&/@Range[2,Length@list]