Originally posted by Puck
Yak-3 is pretty much a variant. More metal than the -9, some minor shape changes, but you could argue the same about an F4U-1/1A/1C/4...just minor tweaks and good skinning. I'm translating the pilot's manual and cockpit diagram for the -3 now, just because it's cool (slow going...adding new words to my vocabulary).
I always thought the Yak-3 (a late model that came after the Yak-9) was significantly different: lighter and more manouverable. Indeed it is called a "light" fighter as oppossed to the longer indurance "heavy" fighter Yak-9. Squadron pub 1078 gives the following specs:
Yak-3: Empty weight 4642 lbs, 1244hp, top speed 404 mph
Yak-9D: Empty weight 6107 lbs, 1360hp, top speed 374 mph
Armament is nearly the same (Yak-3 has two 12.7 mm vs 1 for the Yak-9D- both have one 20mm ShVAK).
You are right, however, they look very similar and visually it is just a variant. Performance wise, however, the Yak-3 would be a significant improvement.