Modalist
One person in three modes or operations — Sabellian; rejected by II.3's 'three persons of one substance.'
This is a contested or rejected alternative.
The Westminster baseline on Trinity is Nicene-With-Filioque. Personas and schools listed below hold this alternative position instead — either because they argued for it at the Assembly (like the Erastians on polity) or because they represent a receiving tradition that departed from the Standards on this point.
Other positions on Trinity
Nicene-With-Filioque WCF
One God in three persons; the Son eternally begotten of the Father, the Spirit eternally proceeding from the Father *and the Son* (WCF II.3).
Eastern-Procession-From-Father-Only
Spirit proceeds from the Father alone; the Eastern Orthodox position rejected (implicitly) by WCF II.3's filioque.
Subordinationist
Son or Spirit ontologically subordinate to the Father — Arian/Socinian; rejected by II.3.