God as One and Three
Moving to the Trinity is kind of like going from considering God on the outside, to considering Him from within. Trinitarian knowledge was not available to Old Testament believers, as it is to us. There are mysteries here we can't figure out, but also reward as we ponder. It is in Christ's cross that we get closest to knowledge of Father, Son and Spirit (John 13-17).
God is one, and the bible also says Father, Son and Spirit are each God.
God is one
There is only one God, and He is unique (Deut 6:4; 32:29; Isaiah 44:6-8; 45:5-6). Because of the first commandment, this isn't just esoteric theology, but practical piety.
What about the Bible speaking of other gods (Ex 15:11; Judges 11:24; 1 Kings 18:27)?
These texts make the point that they are less than God, or they speak ironically or sarcastically. The whole Bible affirms there is no one else equal to God (Genesis 1:1).
Moltmann is wrong to define monotheism monistically (excluding any idea of plurality). He seems to do so to reject God as ruler and Lord over all, which rejection is unbiblical.
God is Simple
God is not made up of parts. His attributes are not parts. This doesn't rule out all complexity. There are "distinguishable aspect[s]" of God. Aquinas was wrong to over emphasize God's unity to the detriment of His plurality.
When God swears by His own holiness, He is not swearing by some part of Him that is less than Him, but by Himself (Heb 6:13). Without any of His attributes He would cease to be God. That doesn't mean the attributes are synonymous. They describe a Person, which is the Biblical way to depict God, and to express the simplicity of God - better than scholastic methods.
God is Three
We can regard God as a plurality, based on Rev 1:4; Gen 1:26; and others.
Some attributes of God are personified things. The word of God, wisdom of God, name of God and glory of God. Many of these are incorporated into the Trinity in the NT.
The Spirit and Angel of God are Divine Persons revealed in the OT (Gen 1:2; 16:6-13). The Angel is sometimes distinguished from God,, but not always.
Frame also sees Messiah in this category, but I can't see that the OT insists that Messiah will be divine. There are hints in Isa 40:10-11 and Ezek 34:11-16, but Psalm 110, which he references, doesn't prove it conclusively. To be greater than David doesn't require divinity.
The OT has some triads that hint toward Trinity.
Aaron's blessing is threefold (Num 6:24-26), God is sung to be thrice holy (Isa 6:3), He has a threefold office (Isa 33:22). God, His Word, and His breath are all mentioned in His creating the world (Ps 33:6). Messiah says that "God has sent Me and His Spirit" (Isa 48:16).
The NT continues speaking of one God, not adding two more, but sometimes applying Scripture speaking of God to particular Persons of the Trinity. Trinity doctrine isn't laid out, like justification is, because there was no dispute about it. It is assumed, since they experience the Incarnation, Resurrection and Pentecost events. Jesus' Incarnation is by the Spirit, as is His whole ministry. He speaks of union with Father in the upper room. He tells us to baptize into the Triune Name. Jesus ascends to the Father, who gives Him the Spirit to pour out on us (Acts 2:33). Romans deals with Father, then Son, then Spirit in the first 8 chapters.
Each Person is involved in our salvation and is to be worshipped.