Arma is originally a Latin word:
arma (genitive armōrum); n pl, second declension (plural only; not used in singular form)

1. defensive arms, armor/armour, shields, weapons of war.

2. war

3. soldiers, military power

4. defence

5. tools

ArmA is replacement name of games successing Operation Flashpoint (name Operation Flashpoint belongs to publisher Codemasters ) so ArmA 1 is Flashpoint 2 that cannot be called Flashpoint, same goes for ArmA 2 and 3.

ArmA is also originally meant as acronym of Armed Assault (used only in context of ArmA (1): Armed Assault )

ArmA 3 is ultimate open-world war simulation game by czech game developers Bohemia Interactive
James: Lets play my new spledid ArmA 3 Zeus mission

David: Wait, I m downloading gigabyte of new update
by ItIsOurJobToRespond June 19, 2015
Get the ArmA 3 mug.