I bought the 3C recently so I could have variety to grind out basics so that I can get my 3M kitted out. However, I've been finding myself actually doing better in the 3C than my 3M!
I run mine right now with the 320XL that came with the 3M (unfortunately, I do not have the funds for a 340XL right now). Weapons are 4 machine guns, and either a large pulse laser or a ER large laser.
And get this, since I bought it with the intention of just grinding out basics, it has no weight upgrades and is still rockin single heat sinks! If I continue to have success with it I might as well drop the cbills for the upgrades and master it! But right now, I still need money for my third chassis.
Anyway, with the machine guns, understand that they are next to worthless against a target that is still armored. Should you choose to run 3-4 machine guns, your playstyle will be dictacted by the energy weapon(s) you carry.
-With the ER large laser, you will be limited to a pretty inefficient sniper build (Raven-3L, Cicada-3M, and Spider-5D are better for this role).
-With the large pulse laser, you are a little closer to the battle and will have to take pot-shots while making sure lights and streakboats don't get close to you.
-I personally haven't tried multiple medium lasers and ERPPC yet because it'll probably get too hot for my single heat sinks. I've also seen some builds that will compromise on the engine size to fit an LBX in there for moar crit power. Might give that a try later.
Regardless of your energy weapons, what you do with your machine guns is the same: destroying exposed internals. I can't stress enough how bad machine guns are against armor. They simply do not do alot of damage, and the only redeeming value really is the crit chance. So, if you see a weak point, do a quick survey of the battle and make sure you have an exit route. If everything looks good, make your run. You'll want to avoid charging straight into your target unless you know that they can't fire back. Make sure you approach at angles where you have enough twist to keep the target under your crosshairs. Don't stop to fire at an enemy unless it's a stomp scenario.
Here's a game a had a couple days ago: 3 kills, 8 assists, 444 damage. I somehow made over 200k cbills too, without any sort of bonus or consumable. As you can see, it was a close game, and I didn't make it to the end. But, I certainly pulled my weight. 400+ damage is decent for this kind of build (again, your machine guns won't give you a lot in terms of raw damage), I average just under 300 damage with it. You will be very reliant on your allies in this build. Your job is to quickly take out hurt mechs. The pug lottery has been pretty good to me the past couple days, so always keep that into consideration.