Answer:
Principally, and most urgently, to make it impossible for Britain to support the Confederacy without looking pro-slavery. (This was successful)
Also, hopefully, to restore Northern morale by making them feel they were fighting a noble crusade to free the slaves. (This was less successful, as reflected in the results of the mid-term elections.)
I don't think you can find three.
There were really two:
Urgently - to keep Britain from helping the Confederates, by making them look pro-slavery if they did.
Long-term - to give the war-weary North something nobler to fight for than just Unity (or the cotton revenues) by turning it into a morale crusade.
Most urgently, to keep the British from sending military aid to the Confederates.
By turning the war officially into a crusade against slavery, he made it ethically impossible for the British to be seen fighting on the side of the slave-owners against the Abolitionists.
Lincoln was also hoping that this new crusading agenda would rally Northern morale, which was in a bad state following Lee's dramatic success in the Peninsula campaign. But the Northern public seemed to remain apathetic. Historians have found very little evidence that Northern troops were inspired to greater efforts by the moral imperative of Abolitionism.
To encourage Northerners to keep fighting against the South; also to make it politically impossible for France and England to recognize the Confederacy as a new nation.