We would only ever use 'three million'. Unless you are saying something more general like 'He has millions'. But if you are using a number in front of it, then it's always 'million'.
I'd say that 'millions' can be used if you're counting sets of one million. As in, 'three millions' could refer to 'three sets on one million'. As stated before, it is more common without a number in front of it, as in 'he has millions.'
The weird thing is previously it said that MILLIONEN was billion! I thought it was weird, did anyone else see that?
I found it, BILLION is MILLARDEN, MILLION is MILLIONEN - similar type of words, no wonder I was pretty confused.
Well, shouldn't this sentence have introduced with 'D' ? It's the beginning of the sentence.