Triple Darts is the 3rd upgrade of path two for the Dart Monkey. It makes the Dart Monkey shoot 3 darts at a time instead of one. It may help a lot early on depending on what towers you can buy. Its icon appears as 3 purple darts. This upgrade allows the Dart Monkey to pop up to 12 bloons (4 per dart).
A lot of these combined with some Juggernauts (for Leads) or with Splodey Darts premium you can beat most tracks on Easy or Medium difficulties and some on Hard.
These are helpful with Splodey Darts: In fact, you can beat most tracks on any difficulty by spamming 2/3 dart monkeys if you have Splodey Darts. Tier 3 Dart Training Facility is very helpful with this strategy.
With the Dart Training Facility Specialty Building, you can get 2-3 Dart Monkeys for roughly 1100/1200/1400 Money with a paid tower and 950/1050/1250 Money with a free tower, being Easy, Medium, and Hard difficulties, respectively.
This tower is better on the mobile versions of Bloons TD because the darts are much larger, meaning it will pop more bloons most of the time.
In BMC Mobile, you have a chance that you get a hint that tells "Dart Monkey's triple SHOT upgrade ". This was probably unintentional. This has been patched, as it now reads, "Dart Monkey's triple shot upgrade is very powerful. Research it as soon as you can."
This tower is surprisingly useful in Battles (especially in mobile), due to its capability to deal with both spaced and grouped bloons effectively. Because of this it's been nerfed 3 times via game updates, making it one of the most nerfed upgrades in the history of Bloons TD Battles (along with Supa-Vac).