Validation rule on COPA assignment

Hello,

I have an issue regarding revenues Account (7* G/L accounts) to be posted manually (FB01,FB60...).

For this posting only one Cost object (WBS or IO or cost center) is needed.

The problem is that I can not define a Field status group who require only one assignement, either WBS or IO or COPA.

Thus, a validation rule creation is needed to check that there is one CO assignement.

How can I create this validation rule ?

Thanks for help