What period of time did McClellan have 3 stars? And did he actively wear this in the field? I don't think I've ever seen a photo w/ him wearing 3 stars.
During the period of General McClellan's active services in the war, there were only two general grades in the U.S. Army; brigadier-general (one star) and major-general (two stars). The rank of lieutenant-general (three stars) had been retired after General Washington held it in the late 1790s.
The custom was thereafter to appoint a "major-general commanding-in-chief" over the army. So while the rank of lieutenant-general was retired, the duty, and the three-star insignia remained. From 1857, that officer (General Scott) wore three stars:
Same in the revised regulations during the war.
General McClellan was appointed a major-general of Ohio Volunteers from civil life (he had resigned from the US Army in 1857 with the regular rank of captain) on April 23, 1861, and commissioned a major-general in the regular army of the U.S. on May 14, 1861.
He was appointed major-general, commanding-in-chief of the army from Nov. 1, 1861, and held that post to March 11, 1862, when he stepped down from that post to command the Army of the Potomac in the field in the Peninsula. During that period he was authorized to have worn the three-stars.
And here is an image of General McClellan as major-general commanding-in-chief wearing the three stars during the period November, 1861-March, 1862:
When he took the field in March, 1862, he was consequently authorized only the two stars of major-general. Shortly afterward Major General Halleck was appointed major-general commanding in chief, and he wore the three stars...
Here is McClellan's "field" coat of 1862, with the two stars of a major-general:
Here he is wearing it while meeting President Lincoln in late 1862, shortly before his retirement from active field service:
Mac resigned from the army on Nov. 8, 1864 (the day of the election in which he was contending against President Lincoln). He was at that time a major-general (two stars).
The grade of lieutenant-general was re-created in 1864, and General Grant was so promoted.
This act was passed on Feb. 4, 1864, and after his promotion Grant wore the three stars as lieutenant-general, and commanding general of the army:
In July, 1866, the grade of "General" of the US Army was created and Grant so promoted, which included the employment of four stars.