Considering Tomb Raider or Bordelands numbers (both 100m), i'd say something in the 100-200m at least (for such a big saga, not any AAA ofc) , would definitely be cheaper if it was just about excluding Xbox, but PC is big nowadays, and you don't just have to pay for the copies that won't be sold but for the players the saga is losing, the more sales a game has, the easier for future games to do better numbers.
About the 3 million sales these aren't terrible, even for a main Final Fantasy game, bit weak compared to the 5m day 1 from FFXV but it was expected, i think the saga will make a comeback with FF 7 Rebirth