Most Christians have just one god, but they see the one god as having three divine personages: the father, the son, and the holy spirit. These three persons are distinct yet coexist in unity, and are co-equal. This is similar to the Hindus who have one creator god Vishnu and the other gods are simply aspects of Vishnu. Unitarians differ from other Christians, they believe in one deity who is one person. The United Church of Canada has indicated in the past that Jesus is not divine.
The role of the Christian saints and angels is problematic. Many religions with a pantheon of gods have a larger group of demigods with significant godlike powers. Whether these saints and angels fill this role is difficult to determine even though they seem to have various protective and supernatural powers.
Christians believe there is only one God.
They were non Christians. They believed in many different gods. Not in just one god.
Christians believe in only 1 God.
Early Christians refused to worship the Roman Gods. Virtually every other religion at the time was polytheistic (many gods), so when
Christians refused to worship Roman gods.
Christians believe in God and Jesus
Jews and Christians
Pagan Romans believed that there were many gods, and wanted the Christians to worship or at least recognize Roman gods in their worship as well as the Christian God. The Christians wouldn't do this, because it went against their religion to worship more than one god. The pagans also wanted the Christians to do ritualistic sacrifices to their gods, which the Christians also refused to do, as well as worship the Roman emperor. Not worshipping the Roman gods, not sacrificing to the gods, and not worshipping the emperor were all crimes of law in Rome, and so many Christians were killed for not doing these things. Christians were also accused of cannibalism (this is my body, eat this is remembrance of me...), and not contributing to society/laziness.
Egyptians believed in multiple gods until around 200 CE when they began to become monotheist Christians.
Christians refused to worship Roman gods.
There is no real answer to this. Christians worship God and Jesus and Hindus worship a plethora of gods, one god, or no gods at all.
No Christians believe that there is one God and that God can do anything so they don't believe that there needs to be three gods.