2017-09-29 : Once you’ve attacked with three or more creatures, Legion’s Landing will transform even if some of those creatures leave the battlefield or are removed from combat.

2017-09-29 : The last ability of Legion’s Landing only counts creatures that you declare as attacking creatures. Creatures that enter the battlefield attacking won’t count.

2018-01-19 : For more information on double-faced cards, see the Ixalan mechanics article (http://magic.wizards.com/en/articles/archive/feature/ixalan-mechanics).