Here are some sibling name suggestions for Campbell, Scout, and August, keeping in mind a mix of girl and boy names:

Considering a Classic/Slightly Rustic Feel to Match Existing Names:

• Girl: Clara, Willow, Hazel, Gemma, Eloise, Juniper, Theodora
• Boy: Jasper, Arlo, Felix, Theodore, Silas, Rhys, Owen

Considering More Modern/Unique Names:

• Girl: Indigo, Wren, Lyra, Athena, River, Juno, Calla
• Boy: Atticus, Ezra, Milo, Asher, Arden, Brooks, Finn

Considering Names with Similar Sounds/Syllable Structure:

• Girl: Calliope, Scarlett, Celeste, Louisa
• Boy: Augustus (variation of August), Caspian, Sebastian

Considering Names with Similar Initial Letters:

• Girl: Cecilia, Catherine, Caroline
• Boy: Caleb, Cole, Cameron

Important Note: The "best" names are subjective. Consider the flow of the names together when said aloud, the potential for nicknames, and the overall feel you want to create for your family. This list provides a starting point; further research into the meanings and origins of names you like is recommended.