France, Poland, and the Soviet Union used the Yak-3s.
Luftwaffe Ordered to Avoid Combat
The Yak-3 was found to be extremely maneuverable in close in fighting that took place below 16,400' / 5,000 m and by 1944 the Luftwaffe issued orders that pilots were "to avoid combat below 5,000 m with the Yakovlev fighters lacking an oil cooler under the nose."
In one dogfight in July 1944, 30 Bf 109s and Fw 190s went up against 18 Yaks and lost 24 against a loss of 1 Yak.
All French unit fighting with the Soviet Union on the Eastern Front. 99 out of the 273 kills the unit did were with the Yak-3.
After D-Day (June 6, 1944), this unit chose the Yak-3 out of all fighters to take back to France.
Polish Warsaw 1st Regiment
The Yak-3 was also used by the Polish Warsaw 1st Regiment.