Well, I've never seen that claim promoted seriously for the Jews who followed Yahweh. Yahweh has many names, granted, and there are many false gods mentioned in the OT, the point being they are not gods but people worshipped them as such.

Example, in Exodus 3:6 (NIV)

'Then he said, "I am the God of your father, the God of Abraham, the God of Isaac and the God of Jacob." At this, Moses hid his face, because he was afraid to look at God.'

So Abraham, Jacob and Isaac were monotheists. Moses was a afraid to look at God, not to look at "a god". Monotheism.

The reference to the triune creator God as plural is not unsurprising.