I agree. In fact, I think a trinity movie would be the best way to introduce Wonder Woman to the general audience. As much as we fans may want to deny it, the GA really doesn't care much at all for Wonder Woman despite knowing generally who she is. A solo movie for her probably wouldn't do much better than Thor did and WB would undoubtedly see that as a failure.
If they put her in a trinity movie and tell the story from her point of view including Supes and Bats, that's gold. The World's Finest duo could carry the notoriety and sneak WW in to build interest (like the way Spiderman got popular). Then, if the box office results are good, they can move forward with a solo movie for her.
And as said earlier, a Flash movie is imminent regardless of whatever else happens. WB has wanted it for so long and it's now or never for him.