In this scene, Duncan's forces have been attacked by a number of different forces. The forces that are attacking Duncan's are led by the following three men:
- McDonald (or MacDonwald), who was leading Irish troops
- The Thane of Cawdor, who was rising up against his own king
- Sweno, the King of Norway.
We find this out because the Thane of Ross has come to tell Duncan about how the battle has turned out. He tells the king that his forces were in danger for a while but that Macbeth was the one who saved the day.
I would just like to correct the above answer because the thane of cawdor IS macdonwald. Its macdonwald,sweno of norway and a third that is not clearly mentioned