This doesn't add up.
The Cubs are 6 behind Milwaukee, 6 ahead of Arizona. If there's no way for the Cubs to catch Milwaukee, there's no way for the D-Backs to catch the Cubs.
The Cubs are 2 games behind their Pythagorean expectations. The Brewers are expected to be one game better and the D-Backs are expected to be 3 games worse. The Brewers have a +161 run differential, the cubs a 115, Arizona, +9.
Sooo... the Cubs *should* be 5 behind Mil and 11 ahead of Arizona. Projections suggest Milwaukee and the Cubs will finish out a little better than their current win percentage, and that Arizona will finish out around .500.
You really have this backward.
The one thing you have right is that the head-to-head games matter. The Cubs have 7 head-to-head matchups remaining with Milwaukee. If they could "basically take AZ out of the picture" they could just as easily reel the Brew Crew back in.
What is likely is that Milwaukee and the Cubs remain on their current top 5 team in baseball trajectory and that AZ remains slightly above average and just out of the playoff picture, regardless of the outcome of this particular 3 game series.