You combine 3 pieces of ammo into one massive magical weapon.  Make a ranged spell attack against the target. On a hit the target takes 3d8 piercing damage.  If they are within 10ft of a wall or structure they must make a strength saving throw or be pinned to the wall and grappled (your spell DC).  They can use their action on their following turn to get off the arrow but take 1d6 more damage.