Here is the way I would do it. Let's say, to expand your example, you had the values 3, 25, 32 in A2:C2. In A1, place this formula:
=A2&"("&TEXT(ROUND(A2/SUM($A$2:$C$2),2),"0%")&")"
The meat is the TEXT(ROUND(A2/SUM($A$2:$C$2),2),"0%"). This will divide A2 by the sum of the three columns...