There are 3 Diglett to find in Warm-Up Tunnel.
Alolan Diglett #1
This Diglett can be found hiding against the tunnel’s wall. When you enter Warm-Up Tunnel from Training Lowlands it is by the entrance to your left.
Alolan Diglett #2
Halfway through the tunnel there is a path that leads to a dead end and the item TR76 Stealth Rock. When you head back onto the main path you’ll see that there was a Diglett hiding behind the two rocks.
Alolan Diglett #3
The Diglett is just before the entrance to Potbottom Desert. It is by the tunnel wall.