So im at work thinkin about what this third ability would be and i land on the tower sheild, tall and sturdy as hell and thus came to be this concept:
Towers Mortar is a tall but somewhat narrow sheild that is summoned where the titan is facing. When summond the sheild is locked into position as a nigh invincible cover with the titan stuck in place behinde it. While locked the titan gains continuous ammo regeneration and can fire at the sheild to "feed" ammo rounds to the sheild itself which fires them at the enemy where the titan is looking.
Firing sniper, linier rifle, pistol, sidearm, shotgun, auto rifle and machine gun rounds will make the sheild fire void bullets.
Firing rockets or gernade luncher rounds will make the sheild fire explosive void bombs with slower rate of fire
Firing a tracher rifle will fire a void laser but it will be slow to aim
If Teamates behinde the sheild fire at it with a weapon corresponding to what the titan is firing, it will boost certain traits.
For bullets it increases the rate of fire
For the mortar it will add more explosive rounds to a single shot, making it a cluster
For the laser it will make the beam larger in size
This is just what i could think of at the top of my head and could use some tinkering but i belive in the concept