When Legion's Landing enters the battlefield, create a 1/1 white Vampire creature token with lifelink.
When you attack with three or more creatures, transform Legion's Landing.
For more information on double-faced cards, see the Ixalan mechanics article (http://magic.wizards.com/en/articles/archive/feature/ixalan-mechanics).
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.
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.