Tri-Pass is an item in the “Plot advancement” category (Key Items).
In-game descriptions
| Emerald | A pass for ferries between ONE, TWO, and THREE ISLAND. |
|---|---|
| FireRed / LeafGreen | A pass for ferries between ONE, TWO, and THREE ISLAND. It has a drawing of three islands. |
Where to find it
Not placed on the atlas maps (so far: Red/Blue and Generation 3).
Sources : PokeAPI (items, item_flavor_text, machines) · atlas · pokeemerald species_info.h