Add Item Category KFF into custom Form

another requirement.  I have a user want select item category one segment a time in a custom form.  So I decide to use the Item Category KFF.  The KFF have segment 1, segment2 and segment3.   When the item category combination ( seg1.seg2.seg3 ) is not valid, error message.  That is all good.

But now user want to change a bit.

  seg1 and seg 2 must be entered.  Seg3 can be NULL.  But if user only entry seg1 and seg2 and leave seg 3 empty, only validate the category seg1.seg2  only.  But if seg1, seg2 and seg3 are all entered, valid them just the old way.