The other two girls are from this tweet by Tatsuki; they don't have names yet. https://twitter.com/irodori7/status/951836878155558912/photo/1

Here's the video the other two girls are from, titled へんたつ (Hentatsu): https://www.youtube.com/watch?v=9hCRCXn0Cwc

I found a couple of JP pages rounding up what the "investigation squad" has figured out so far.
http://irodorich.com/archives/130
http://kai-you.net/article/50064

Fans theorize that the oni girl represents Tatsuki, while the animal ear girl represents Isa Yoshihisa, another member of irodori. Isa is credited in irodori's works as 平安, which is usually pronounced Heian, but is pronounced Hen in Okinawa; Isa is an Okinawan family name. This would make the title of the video a reference to Hen and Tatsuki.

The oni girl has "07 24" on her horns (see the very first scene of the video). This is theorized to mean July 24, the date of Tatsuki's Tweet about Kemono Friends season 2: https://twitter.com/irodori7/status/889477890420523009 (post #2823867).

Oni girl is voiced by Takasugi Kaoru (Tweet), animal ear girl is voiced by Kadowaki Mai (Tweet). Kadowaki refers to her character as 猫さん (neko-san/cat).

The cafe is a real place in Nakano Broadway, called A License.

A new Hentatsu short came out the other day, and it seems I had missed one, so here's a list of all of them so far:

Part 1 - Hentatsu (YouTube) / (Twitter). 1:56
Part 2 - Hentatsu Continued (YouTube) / (Twitter). 2:20
Part 3 - Hentatsu Continued Continued (YouTube) / (Twitter). 2:10
Part 4 - Spring Relaxation (Twitter). 0:33. No dialogue
Part 5 - A bit of a continuation (Twitter). 1:13
Part 6 - Just a bit more continuation (Twitter). 1:36

Edit: Added part 6.