Ward or McSally can win in November. Arpaio can't. Simple as that.
Arpaio has zero chance. Ward has issues. She's a risk. McSally has the best shot to beat Sinema.
After the Roy Moore debacle, I don't think Team Trump want to take chances. All the President wants is a "yes" vote. Yes on judicial nominees. Yes on agenda items. He'll get this with any of the three (R) candidates so it's best to back a winner.
President Trump will endorse McSally.