Triple Attack is a devastating attack that may destroy three groups of units with just an attack. When
you use a group having
Triple Attack to attack, the adjacent unit groups will get
50% of
the damage inflicted on the target group.
This attack is ideal to destroy the enemy's
Cannon Fodder and also to reach
unreachable groups.
Here's an example of the attack:

All the enemy's
Rain will be destroyed by that
Driller. That's because the
Driller has
Triple Attack
and because the
Rain groups are on exactly that positions, adjacent to the target, and perpendicular
to the
Driller.
Combat Units with
Triple Attack are also able to reach
unreachable groups. Consider the folloging
example. The enemy's
Fenix is well protected by
Cannon Fodder, that prevent a direct attack.
However, once again, the
Driller can sneak in and inflict damage on the
Fenix using
Triple Attack:
