How does Armamentarium interact with Surplus?

The Surplus perk gives increased handling, stability, and reload speed for each of your three cooldown abilities (grenade, melee, class) that are fully charged.

Armamentarium is an Exotic Titan Chestplate that gives you an extra grenade charge.

I see three possible scenarios, but have no idea which one it is. I have Armamentarium, but no Surplus perk weapons and I want to know if it's worth farming for one.

1. Surplus only procs for the grenade ability when you have both grenades full. (Meh.)
2. Surplus procs for the grenade ability as long as you have one grenade ready to throw. (Pretty good, actually.)
3. As with #2 but also Surplus will stack to x4 if you've got both grenade charges full. (This could make Eriana's Vow not at all cringeworthy to use as a main weapon.)

Solution 1:

According to this Reddit thread, which breaks down the benefits received from Surplus, states that the boosts cap out at 3 stacks. If you were to have a fully charged melee, grenade, and class ability in addition to a fully charged secondary melee or grenade, you wouldn't receive a 4x or 5x boost, but only a 3x.

Any combination of fully charged abilities contributes to the stack. So x2 grenades and x1 melee has the same effect as x1 grenade, x1 melee, and class