I would sell the older two, then buy two more 18 years, preferable with all star+ potential, and 4000 plus salary, center skills primaries (IS, ID, RB and SB) skill points totals of 22-28. then look for desirable secondaries, a little passing, a little handling, MAYBE some JR and JS. definitely try to avoid atrocious, anywhere.
You can try drafting them, but its more of a sure thing to try to buy the ones you want on the TL. I would train all 3 in single position, rotating between IS, ID, RB, and SB, try to keep them even, you will train faster (as you avoid the elastic effect), and it keeps salaries down. Now, ALOT of people don't like SB, but i bet it will cost more money in the future (when you get ready to sell, because people will still be trying to stop LI teams.
Buena suerte.