Civil War is so far 3 millions ahead of Iron Man 3 after 31 days of release.
However its dailies are a little lower (by about 8% over last week end). We should also not that Civil War managed a significantly better post memorial day week end drop than IM3 which is an encouraging sign moving forward and the film could very well be pulling ahead in dailies next week end.
IM3 went to make another 24M dom after its 5th week end.
Assuming the gap in CW's dailies stabilises around 8% lower than IM3 for the rest of its theater run, that's another 21M or so for a finish right around 409M, tied up with IM3.
If the gap gets smaller (like this week end might suggest) and Civil War catches up on IM3's dailies then it can do 412M +.
It's all gonna be heavily dependent on how long Disney intends to keep it in theaters and how they are going to play its second run in dollar theaters but I think its final domestic gross should land anywhere between 406/413M with still a chance at outgrossing IM3 (domestically).
So yeah, long story short, Minions is going down anyway.