Well, if its any defense for the La-7, the 3-cannon version was rare indeed, but only because the introduction of the advanced, B-20 cannons were late. After 1945 the 3-cannon version was to be considered a new standard. All La-7s after that time would be equipped with 3 cannons.
Much simularly, the Bf109s armed with MK108 30mms, were also part of a standard armament option after late 1944. In case of the Fw190A-8s, the MK108 armament seems a bit more pronounced as a specialized "buff hunter" variant, but these Sturmbock conversions were themselves fairly numerous.
With the P-38 and P-47, people in the MA usually fly with the increased ammo option, and IIRC, this is the "extra ammo" option. The basic option would be to fly with the smaller ammo load, but for both those planes, increased ammo was also a part of a standard armament loadout.
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I'm not sure about the Hellcat, but my impression is that it's not anywhere near something to be considered a "standard armament option"... it was available only for night fighters, is it not?