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.
I need help, badly!

I need help, badly!

What is this? … last one I promise!

What is this? … last one I promise!

You all were right.

You all were right.

the carrots wear the pants in this family

the carrots wear the pants in this family

Curtains!

Curtains!

A few lasts for this season.

A few lasts for this season.