set = 3 pieces(different size)

if we have item uom is set, and consist of 3 pieces different size(110-100-90).

so we will define item x primary uom is set and secondary is piece and rate is 3.

but i have a problem i define 3 without size size(110-100-90).

we can manufacture item x.

we can sell  item x.

we can sell item x with secondary uom but when i sell i don't know the size of piece.

what is the solution ?