More like "extremely likely". Those two seemed super excited to see their friend finally decide to join in.
There may be some trouble in that threesome, tho. Shibuya doesn't seem at all interested in Kubota. Her feelings are more like desire/interest/attraction towards Tada, and rivalry towards Kubota.
Oh and also: LOL
Not sure if I entirely agree with this assessment but let's say that's true, it doesn't mean you can't fuck your rival :P
Plus I mean if your friend is already having sex with your other friend who is also your crush...seems like the best way to win is to just join them. Everybody gets a piece of the action and in an ideal world everyone goes home happy :D