"Tenemos tres millones de dólares."
Translation:We have three million dollars.
The word 'Millón' is a noun. When you want to use a noun to describe the quantity another noun like this, you use the word 'of'.
i.e. I have 3 buckets OF fish.
It's the same thing, only English has this weird exception to that rule with 'million'. Spanish does not. Of=de.
Tried "$3 million". Every time before this one Duo has accepted a dollar sign and the number. Why change that pattern now?
It simply hasn't been added to the accepted list yet. Report it if you want to.
'three millions' should be accepted (least wise as used in Britain).
Just because a way of saying something has become less popular over time it doesn't make it wrong.