I do not think that God changed from two to three in 381. It is not as if the idea of the Spirit of God suddenly popped into their heads as being distinct but also being God. Scriptures did not change either. After some broad recognition and agreement of Jesus as part of the Godhead had been established in earlier conflicts, where the nature of Christ, as the predominant problem to be solved in earlier years, the resolving of conflicting idea of the Spirit were finally addressed. It seems likely that many of the same reasons for viewing Christ as being in the Godhead also reflected reasons why the Spirit was to be treated equally. But you probably already knew that.