How do Manaforged Arrows Support work?

I'm using caustic arrow( with other support cost 21 mana ) as main skill while manaforged link to toxic rain( 9 mana ). Usually it take 3 shots to trigger but sometime it take 4 shots

1) The 300% mana cost is referring the main skill?
2) Do reduced mana cost help? Or they scale together?
3) Is there anyway to trigger every single shot?

1) 300% cost is referring to the final mana cost of the skill that would be triggered. The Toxic Rain you mentioned has 9 mana cost from the gem, and Manaforged Arrows Support applies a 200% mana cost multiplier.

With no other modifiers to its mana cost, you trigger Toxic Rain every 54 mana spent on other bow attacks while Toxic Rain is not on cooldown. Manaforged Arrows Support also gives the linked skills a 0.5 second cooldown.

2) % Reduced mana cost will affect both equally, flat -mana cost would have a larger impact depending on which has a lower mana cost.

Keep in mind both of these would also affect the damage multiplier that Manaforged Arrows Support grants, if you're interested in that.

3) High mana cost on your main skill, as well as not exceeding 2 attacks per second (though this would vary depending on your cooldown recovery rate).