Slightly convoluted situation here: Sekaiju no Meikyuu 2 never got its own tag since it's an extremely similar game to the first MeiQ, with a new labyrinth and the addition of a couple of extra character classes such as gunner and doctor magus. If it were a PC game, it might be considered an expansion pack rather than a true sequel.
However, in April of this year Atlus gave us sekaiju no meikyuu 3 which has a fairly distinctive setting, some novel gameplay elements, and a completely new cast of classes. I assume this is why it's gotten its own tag.
Danbooru's usual policy would be to umbrella-tag the series- moving posts to sekaiju no meikyuu 1 or whatever's appropriate, then implicating all games in the series back to sekaiju no meikyuu- but this doesn't really work when the first two games are held to be synonymous. (Since SQ2 has all the same classes as SQ1 did, there's no way to prove an image is based on the first game and not the second, unless it contains a characteristic NPC or predates the release of SQ2.)
Our other options seem to be:
- alias sekaiju no meikyuu 3 to sekaiju no meikyuu and consider the whole series a monolithic enterprise
- implicating sekaiju no meikyuu 3 to sekaiju no meikyuu (pretty much the status quo, most SQ3 images are being tagged with both)
- remove the tag sekaiju no meikyuu from sekaiju no meikyuu 3 posts, holding it to be a wholly separate game from the first two
I'm in favor of implication, personally; the only consequence is anyone specifically searching for SQ1/SQ2 posts would have to subtract SQ3 from their tag search, but that doesn't seem likely to be a major concern. However, I thought I would try to lay out the whole situation and explain why an umbrella tag probably will not work.
Updated