If we are talking about USPSA bases 3-Gun/Multi-gun rules then:
If you shoot Open
then you will have Shotguns with speed loaders (or magazines in the case of the Saiga) the may also have compensator and/or ported barrels and optics and 10+1 capacity. Rifle will have optics potential multiple optics (Scope and red dot or similar) have larger compensator/muzzle breaks then allowed in Limited and can have a bipod mounted. The handgun will follow USPSA Open Division, comps/ported barrels, optics, 170mm magazines and 9mm or larger allowed for Major.
If your shooting Limited
then you cannot have optics on any of the guns. Shotguns cannot be compensated or ported, no optics allowed, no speed loaders or magazine fed shotguns, and capacity is limited to 8+1, With rifles compensator/muzzle breaks are allowed but must be under a certain size (length and diameter are specified), no optics and no bipod or similar rests. The handgun also follows USPSA Limited rules, iron sights only, 140mm magazines, 40cal or larger to make Major.
There is a Tactical division
that follows all the rules of Limited except allowing one optical sight on one of your weapons. Most shooters choose their rifle.
There is also allowance for other divisions with the most notable being Heavy Metal
that requires a 30cal rifle, Pump 12 shotgun and a 1911 pattern handgun in a major caliber (40cal or greater).
The first three division are usually recognized at most matches. The Heavy Metal division is not always recognized. There are a few other more obscure division occasionally seen.
I started with a Browning BPS Stalker. A pump gun is not as fast as a semi but since the BPS does not have a lifter gate due to its bottom eject configuration I can load the BPS far faster than any other shotgun I have seen. Just flip the gun over grab two shells, drop one shell in the port and then shove them both home with the second
I have since picked up a cheap used Remington 1100 and fixed it up and made a 3-gun shotgun out of it. Runs good and I am slightly faster shooting it but its still slower loading than my BPS.
TOP: Browing BPS 3.5 inch Stalker, 26inch barrel (Inv Plus tubes), homemade magazine extension (9+1), and fiber optic front sight.
BOTTOM: Remington 1100, 26inch (Rem Tubes), Dave's Metal Works magazine extension (10+1), DMW easy loader gate, Choate Combat bolt handle, William's clamp on rib fiber optic rifle sights.