MiG 3 is maybe my favorite early war VVS fighter! It must be handled very carefully especially when slow IAS but the reward is ability to not be pwned by 109's.
Mixture should be made leaner when you hear the engine begin to run rough. I used the + and - 5% keys but slider or rotary would be even better. I am no expert, I think that would take flying nothing but the MiG for a long time and a good ear for engine rather than memorize heights. Besides, IL-2 atmosphere up to 10km is modeled so well that summer from winter makes a difference.
Also you may notice dark exhaust when running too rich. Better late than never.
But don't forget to set mix back when descending!
It has no supercharger gear changes yet performs better at high alt than low, there must be a compressor?
Other major things are keeping the flight 'coordinated' (slip ball centered) and trimmed for straight and level flight.
Are you using WEP? Without WEP you must be very good to reach 600 kph in the plain MiG 3.