ok, im in desperate need of help...

i have 20 columns (cells A1 to T1) in these cells the default value is 'x' but these x's change to numerical values when i enter data elsewhere. leaving me with a possible example like this...

12 4 4 11 7 1 1 19 20 6 13 x x x x x x x x x

i want to be able to add up the sum of the last THREE values (on the rightmost side) that arent equal to x

i.e. i want to add 20, 6 and 13

however the position of these last three values can vary between anywhere in these columns although they are always joined (never an x between two values but can be in columns A,B,C or G,H,I or R,S,T)

and i cant change the order the numbers appear.

anyideas what i could enter into cell U1???