I think if Grealish is to play they would have to drop Foden. I don't see how you can have Grealish, Foden and Kane all looking to ball hog. Even if Kane is told to adjust it does not look right.
I think it would have to be Grealish on the left, and maybe Sancho on the right, or Sterling. You can't have Grealish, Mount and Foden in one team in my opinion, too many players with the same strengths - that is very easy for the opposition to prepare for. There's not enough forward runs or stretching the opposition in this England team in general.
The only way it may work is if one of these players played a genuine CM role, but that's very unlikely. Maybe against Czech Republic you could do it (although Southgate will not) but it's not viable later in the tournament.