New Super Luigi U: Acorn Plains-5 -- Piranha Gardens

Aside from Piranha Plants, there are Hammer Bros in this level. The first Star Coin is under the ledge with the Red Coin Ring. The second Star Coin is near the second Hammer Bro. There's a 1-up in the second brick block from the left on the top level of brick blocks where the second Hammer Bro is. Keep going and you'll see a giant Piranha Plant on the rock above. Enter the hidden area from the left side of the rock for the third Star Coin. Keep going to reach the flag.