Legion will increase its own Attack points by 3 when there are two other Infantry cards on the player's field. If none of the applicable cards on the board, the ability will not activate until they are added, and will deactivate if any become knocked out or ruined. These points will not carry over to subsequent rounds.
A legion was a combined arms major military unit, usually featuring cavalry, infantry, and artillery. Often used in reference to the Roman Legion, usually comprising 3000 men in early times to over 5200 men in imperial times.