All 3 bonus levels are on the other side of the Piraka Garden.
Go back to where the shop and entrance to the levels are, then go through the gate to the Piraka Garden.
Once in the garden go through the archway on the other side of the first area. This will take you through a kind of jungle tunnel into a second area.
Go through the archway on the other side of this area, and you'll get to an entry point to all 3 bonus levels.
- Iain