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.
my room

my room

Need Plant ID

Need Plant ID

My sister’s room

My sister’s room

Everything is in bud on Bloom Day

Everything is in bud on Bloom Day

OH. MY. GOD.

OH. MY. GOD.

~Help needed~ *new room*

~Help needed~ *new room*