3. (If this creature isn't monstrous, put three +1/+1 counters on it and it becomes monstrous.)
When Arbor Colossus becomes monstrous, destroy target creature with flying an opponent controls.
Regler omkring kortet.701.28
“Monstrosity N” means “If this permanent isn’t monstrous, put N +1/+1 counters on it and it becomes monstrous.”701.28b
Monstrous is a designation that has no rules meaning other than to act as a marker that the monstrosity action and other spells and abilities can identify. Only permanents can be or become monstrous. Once a permanent becomes monstrous, it stays monstrous until it leaves the battlefield. Monstrous is neither an ability nor part of the permanent’s copiable values.701.28c
If a permanent’s ability instructs a player to “monstrosity X,” other abilities of that permanent may also refer to X. The value of X in those abilities is equal to the value of X as that permanent became monstrous.
Reach is a static ability.702.17b
A creature with flying can’t be blocked except by creatures with flying and/or reach. (See rule 509, “Declare Blockers Step,” and rule 702.9, “Flying.”)702.17c
Multiple instances of reach on the same creature are redundant.