Part 4: Beat Burgh — Castelia City Gym, badge 3

Beat Burgh — Castelia City Gym, badge 3 opens 3 areas.

Where to go

1. Desert Resort

   Wild: Darumaka 30% · Lv 18-19, Sandile 30% · Lv 18-19, Sandshrew 15% · Lv 19-20, Dwebble 10% · Lv 19-21 +4 more

2. Relic Castle

   Wild: Baltoy 100% · Lv 27-30, Sandile 40% · Lv 18-21, Yamask 40% · Lv 18-21, Sandshrew 20% · Lv 19-20 +3 more

3. Route 4

   Wild: Frillish 100% · Lv 5-15, Alomomola 95% · Lv 5-20, Finneon 55% · Lv 40-70, Clamperl 40% · Lv 40-60 +15 more

Everything you can have caught by this checkpoint · Open the interactive guide

It also doesn't say when 5 of its areas open up, so they are listed together in the last part rather than guessed into the story.