The actor that has three masks, can only wear one mask at a time. That is what this theory teaches. At one time he is wearing the mask of the Father. At another time he is wearing the mask of the Son and at another time he is wearing the mask of the Holy Spirit.
When he ceases wearing a certain mask, for example the Spirit, the Spirit ceases to exist because that mask has been laid aside.
God is one being but eternally manifested in three persona's that are co-equal and co-eternal. God does not put on one mask at one time and then another mask at another time.
Furthermore, these persona's are not simply "masks" but three distinct conscious personalities that fellowship with each other.
Sebbellianism would teach that one man can be a father, a son and a husband all in one person. However, each is only true to a certain audiance. To his wife he is a husband but not her son or father. To his children he is a father but not their son and husband, etc. God is all three all the time to all His creation.