So, here is the A6M3 Experimental heavy cannon fighter. She's fun, but she is no game changer. The M3 suffers from the same shortcomings as the rest of the Japanese line; low top speed, low durability and low altitude performance envelope. If these are things that will prevent you from enjoying an aircraft, I'll save you some trouble and you can stop reading right now. However, if those "features" of Imperial Japanese play in Warplanes isn't a deal breaker, read on, because this aircraft is a lot of fun.
The weaknesses shared with the other Zero models aside, the A6M3 can be a lot of fun, if her pilot is at least competent in going evasive and predicting when and from where they will be attacked. Like the other models of Reisen the M3 can be an extremely evasive and difficult target to hit when played well. Her nearly unmatched pitch ability also allows her to keep her guns on target better than most of her competitors, and half of those guns are 30mm cannons that absolutely shred anything inside of 300 meters and can strike reliably out to 500 meters. Since the M3 cannot climb to the altitudes most veteran pilots hang out in after the initial climb, playing this aircraft means you're going to be flying amongst a lot of bots; both friendly and enemy. Keep your head on a swivel; watch out for overzealous friendlies as you are most likely to get the bad end of any aerial collision. Keeping your eyes peeled also provides the benefit of a forewarning against diving heavy fighters looking to make a grease spot of you in midair.
Since the M3 doesn't have strengths that play into the meta your role in this aircraft is to attempt to swing the numbers game against the enemy team. Your twin 30mm cannons can decimate any fighter you come across before you half overheat them if your aim is decent, allowing you to shift immediately to another target. This process is helped further if your pilot has the Killing Streak (not the actual name) ability that immediately cools your weapons after a kill and increases your maneuverability. The M3 can most reliably affect the outcome of a match by eliminating all low flying bots and enemy players, leaving your team (hopefully) with superior enough numbers to overcome the meta being used by the few surviving players on the enemy team.
A final note on the M3. If you are careful about your angles of approach, don't be shy about taking on Ground Attack aircraft with her. The 30mm guns make quick work of tier IV and V GA and can put a hurting on even tier VI ones. Just mind the tailgunner as you don't have much health to spare.