Threepeater

The Threepeater shoots Peas in not only its own lane but the lanes besides it above and below. These are a handy middle-to-late game weapon, although it is best not to place them in the top or bottom rows, as then they can only fire two peas rather than their usual three. However, it is a good idea to place them there after placing them in the other rows.

Strategy
Since it fires down all three lanes whenever it sees anything in any lane, it can potentially hit zombies just as they enter the right side of the screen - well before other plants have noticed them. It's especially useful in conjunction with the Torchwood and on pool levels, since two Threepeaters in the second and fifth rows (both touching the sides of the pool) can cover all six lanes on a pool level.

Threepeaters have somewhat inacurate animations and can actually hit zombies right above or below them. Because of this Threepeaters work better with torchwood than the animation might suggest: if a Torchwood is planted directly above or below the Threepeater, the Threepeater will shoot fireballs directly out of its top or bottom head (respectively) without the peas even touching the Torchwood. Likewise, if a Torchwood is planted in the row above or below a Threepeater but one column ahead of it, the Threepeater's peas will turn into fireballs despite not having fully moved into the row and not having gone through the Torchwood. Finally, if a Torchwood is planted right in front of the Threepeater all three peas will turn into fireballs.

I, Zombie
Threepeaters are encountered in various I, Zombie levels, including I, Zombie Endless. Like the Starfruit and Magnet-shroom the Threepeater is one of the few plants capable of affected other lanes than the one its planted in. This forces the player to consider other lanes besides the one currently being assaulted. For example: an Imp about to walk down a seemingly undefended lane might still get killed by a Threepeater, wasting sun.

Trivia

 * If there is a Threepeater on the top or bottom row, the top or bottom head (respectively) does not have its firing animation play, as it cannot fire outside the screen which is impossible.
 * The Threepeater is only available in the full version of the game.
 * The Threepeater's three heads bounce at different speeds. The bottom head bounces the slowest, while the middle head has an average speed, and the top head bounces the fastest.
 * Each head also has a different number of leaves on the back; the bottom one has one leaf, the middle one two, and the top one three.
 * The Threepeater is the most expensive non-upgrade plant in the game, costing 325 Sun.
 * The Threepeater has a head smaller than it's projectile.