Full Armor Break

If I put Armor Break 3 and Armor Demolisher 2 gems in my Flash Grenade, will that Fully Break Armor?

Depends on the damage you do vs the armour the mob have.
Some mobs can get their armour broken super easily, where others have to be weared down a bit before it breaks.

Flash grenade seems to scale pretty good with base damage at higher levels, so it might be possible, but it have no default armour breaking in the skill itself.
There is also talents one can take to make armour breaking easier if I recall right.