Cactus ident?

Cactus ident?

Do anyone know what the three-sided cactus that you get in gift sets that often has a pink plant grafted on top is? I have one that was doing really well for a while and now looks a bit worse for wear and I'm not sure what I'm doing wrong. This is probably the best picture I have close up. (Please ignore the katydid.)
 True Northern Katydid
I thought I had found it recently in a botanical garden (Hylocereus species?), but I'm not sure. ETA: I just googled the probable genus name and immediately got success: It's a Hylocereus undatus, the Dragonfruit cactus. That would explain why it's looking so cranky right now - it's a tropical plant and I've had it outside until now. Oops.
Baking with garden

Baking with garden

Healthy Cheap Eats Part I: Parmesan Fish Sticks

Healthy Cheap Eats Part I: Parmesan Fish Sticks

Can you see the rose bush.

Can you see the rose bush.

Des’s Garden pics

Des’s Garden pics

Before & After

Before & After

growing herbs from seed

growing herbs from seed