The collision took place on the middle lane of the three-lane stretch, opposite the Fish Inn on the eastbound carriageway.
A spokesperson for Hampshire Police said officers from both Dorset and Hampshire were dealing with the incident.
Ambulance are also on the scene but any injuries are not thought to be life-threatening or life-changing.
The road is partially blocked but traffic is able to move past, the spokesperson added.
Recovery has been requested for the vehicles.