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]
Alternatively, because First, Rest, Most, and Last are better than Part:
transform =
{Total@(First /@ Most@#),
Sqrt[Total@(Most@#^2)][[2]],
Last@Last@#/((First /@ Most@#).(Last /@ Most@#)/Total@(First /@ Most@#))
} &