The La-5 and YaK-9 certainly have comparable specifications. Also, minor nitpick but the P-51B didn't see service until 1943.
http://en.wikipedia.org/wiki/Lavochkin_La-5
Looking into the La-5 it wasn't really all that good in 1942 and by even 1943 when the 5FN shows up the Spitfire would be superior and might even have issues with the P-47 and a developed D.520, especially RR Merlin or Allison engined. I'm discussing Soviet aircraft on another forum in this scenario and apparently they had a LOT of problems with supercharges and turbocharges, so would be at a major disadvantage above about 15k feet until after 1945. The Su-1 and 3, which I was talking about as a high altitude counter were actually not able to be developed due to their failures in turbochargers and their lag in superchargers.
Wikipedia numbers on service ceiling doesn't tell the full story for Soviet engines, which actually dramatically fell off in power and even 1944 had serious trouble at even 20k feet, reaching less than 900hp at that height, when British Merlin engines were getting over 1300hp. So if RAF and ALA bombers operate like the He177 did in 1944 at 20k feet they could operate with near impunity even as late as 1944 without escort, but with escort there is no threat at all. Mosquitos operating in 1942 and on would be effectively untouchable. The VVS can only compete at lower altitudes.
The Mig-3 would be able to at least contest things at altitude, but if they opted to keep the Mig-3 in production it would badly eat into Il-2 production, because they competed for the same production lines (AM-35 production was discontinued for AM-38 production.
Also the AM-35 engine in the Mig-3 only had enough fuel and power for max 20 minutes at 16000 feet:
http://translate.google.com/translate?hl=en&sl=ru&u=https://ru.wikipedia.org/wiki/%25D0%2590%25D0%259C-35&prev=search
Plus there were serious issues with the engine in production and service, which was one reason it was ultimately cancelled. They would theoretically fix the issues and make the Mig-3 competitive, while building new lines for AM-35/38 production, but in 1941-42 ITTL there will be shortages that might take until 1943-44 to sort out. However in the meantime the P-51B and better Spitfires will be outnumbering and outperforming the Mig-3 in 1942-44.
http://www.ww2aircraft.net/forum/aviation/franco-british-vs-ussr-1942-a-43287.html