The canton Saint-Quentin-3 has 9 communes.
| Code | Name | Population ▼ |
|---|---|---|
| 02691 | Saint-Quentin | 52 813 |
| 02340 | Gauchy | 5 183 |
| 02371 | Harly | 1 569 |
| 02383 | Homblières | 1 412 |
| 02359 | Grugies | 1 280 |
| 02549 | Neuville-Saint-Amand | 848 |
| 02481 | Mesnil-Saint-Laurent | 435 |
| 02142 | Castres | 246 |
| 02214 | Contescourt | 61 |