Except 2 of those 3 have been very underwhelming since arriving?
2 out of 3? Fabinho has been looking really good recently, he just needed 2 or 3 months to get settled, and now he's looking like a classy player whenver I see him play, including today playing in defence. Time will tell with Keita, he needs to mature a heck of a lot though, but he's got the tools. I still think he'll be great in the PL some day!
As for Coutinho, he won't leave Barcelona now after making such a fuss and going to such lengths to get there - he ended up paying some of that huge fee himself to make the move happen. And stated then he wanted to go partly due to his family wanting to live in Spain where culturally it'd suit them more, so moving back to north West of England would seem strange!
Once Barcelona get a new coach, he'll probably settle back into the team a bit more.