It is premature to say Elliott is the top priority because his contract runs out after 2020 and anything could happen in a couple of years. He could do something boneheaded to get suspended. He could suffer injury or just wear down quickly.
If we had to choose among the three, then Elliott is probably a more important piece of the offense than Prescott and Cooper, but I don't see any reason this should be an either/or situation. The contracts can be structured to make all three fit and still have room for whatever we need to do at other positions.
I'd try re-signing Cooper now because it could lower what we're paying him this season, and that money would get absorbed into the overall contract, lowering its value some. I would re-sign Prescott next offseason if he plays like he did in 2016 and the last half of 2018. If the asking price is too much, then I'd franchise him because it would likely cost a little less. With Elliott, I might also try to sign him to an extension before next season, but I'd be very willing to ride it out and see where he's at as a player in two years.