While no Pokemon can be triple-typed by default, it is possible to get there types on a Pokemon.
The move Forest's Curse, introduced in Generation 6, adds the Grass typing to a Pokemon.
The move Trick-Or-Treat, also introduced in Generation 6, adds the Ghost typing to a Pokemon.
Using one of these two moves on a Pokemon will temporarily grant them a third typing, depending on the move you choose, and provided they already were dual types in the first place.