Ah, there are three hypostases and one ousia. That clears it up
If a polytheist says there are 3 gods and one ousia, is he basically in line with Christian doctrine on this?
Well no, because the Christian teaching is that there is only one God. So saying there are three gods would mean there's more than one god, which would not be in line with Christian doctrine.
If not, what is the difference between a god an a hypostasis?
god (gŏd)
n.
1. God
a. A being conceived as the perfect, omnipotent, omniscient originator and ruler of the universe, the principal object of faith and worship in monotheistic religions.
b. The force, effect, or a manifestation or aspect of this being.
2. A being of supernatural powers or attributes, believed in and worshiped by a people, especially a male deity thought to control some part of nature or reality.
3. An image of a supernatural being; an idol.
4. One that is worshiped, idealized, or followed: Money was their god.
hy·pos·ta·sis (hī-pŏs′tə-sĭs)
n. pl. hy·pos·ta·ses (-sēz′)
1.
Philosophy The substance, essence, or underlying reality.
2.
Christianity
a. Any of the persons of the Trinity.
b. The essential person of Jesus in which his human and divine natures are united.
3. Something that has been hypostatized.
4.
a. A settling of solid particles in a fluid.
b. Something that settles to the bottom of a fluid; sediment.
5. Medicine The settling of blood in the lower part of an organ or the body as a result of decreased blood flow.
6. Genetics A condition in which the action of one gene is concealed or suppressed by the action of an allele of a different gene that affects the same part or biochemical process in an organism.
In Christian theology God in His unitary, indivisible being is His one
ousia. The three--the Father, His Son, and the Holy Spirit--are spoken of as three distinct (but inseparate)
hypostases. The Father isn't imagined, but real; the Son isn't imagined, but real, etc. So there isn't an imaginary threeness, but a real threeness in the one, indivisible reality of God's own Self.
-CryptoLutheran