Possibilities
In case all the three teams end up on the same points, the team with a better head-to-head against the other two will be crowned as champions. If this scenario turns reality on May 15, then Real Madrid will finish third, thanks to poor head-to-head record against the other two teams.
Among Barcelona and Atletico, the defending La Liga champions have a better Head-to-Head record.
For Real Madrid to win the league, they should hope that they win all the remaining matches and hope Barcelona and Atletico Madrid draw/lose at least one match.
For Atletico to win, they have to win all their three matches and expect Barcelona to drop points which means, both the Madrid clubs (if Real win all the three matches) will finish above Barcelona.
For Barcelona to retain the title, they just need to win all their matches.
Final verdict: Among all the three teams, Atletico Madrid have an easier set of fixtures. With equal points as Barcelona and a one-point lead over Real Madrid, it is safe to put your money on the Rojiblancos to go all the way to glory in La Liga in spite of Barcelona having a better head-to-head over them.