Forza Horizon 3 Has at least 1.13m players and at least 600k sales. In less than a month!

[I LOVE GIGGS]

From NPD and GFK we know it did 200-225know in US+UK. Since racing games tends to do better outside the US than in it, I think this is a reasonable estimate.
FH3 did very well. Without massive bundling Forza games tends to sell 2-2.5 million copies FH3 will have no trouble passing this number.