That pick they have from the Texans is #3 overall right now. I heard a Dolphins writer on a podcast mention this when Tua replaced Fitz that they wanted to give Tua a test this season because they have the ammo to get Lawrence. There's no way they can be pleased with Tua because despite the media hype he's played poorly. He has three of the worst ESPN QBR games in the league this season.
You're probably asking why the Jags would trade him. Well Lawrence could pull an Elway/Manning and say he's not going to play for the Jaguars. He shouldn't play for the Jaguars. His team should get him to the Dolphins. Tua, #3 overall, Dolphins #1, Texans #2, Dolphins #2 and maybe some future picks is fair compensation.