The M2 Bradley infantry fighting vehicle is a heavy IFV designed as a companion vehicle to the M1 Abrams series main battle tanks. It is heavily armored but comparatively lightly armed, with TOW wire-guided anti-tank missiles, a 25mm chain gun, and a 7.62mm medium machine gun, all in a powered turret mount.
The M-113 is not an infantry fighting vehicle; it is an armored personnell carrier. While it can be modified into a highly capable light infantry fighting vehicle, it is designed as a "battlefield taxi" with limited armor protection. The basic armament is one 0.50-cal heavy machine gun on a pintle in front of the commander's hatch, although improved variants used in Vietnam ususally had an additional two pintle-mounted 7.62mm medium machine guns on the sides, used by infantry riding on top (riding on top in combat was actually more common than riding inside, as it offered better survivability against land mines). The U.S. never really got into IFV variants of the M-113, but various other countries have. Armament is all over the place, with common fits ranging from a 7.62mm Minigun to twin 105mm recoilless rifles and heavy antitank missiles.
Additionally, the M-113 is the most widely used military vehicle chassis in the world; it's the basis of over forty different vehicles, including the FIST-V artillery spotting vehicle, the Chaparral and Vulcan air-defense vehicles, a self-propelled 81mm mortar vehicle...the list goes on and on.
The BMP-3 is a light IFV, just over half the weight of the Bradley. Despite that, it's a lot more heavily armed, with a 100mm low-velocity gun/missile launcher, a 30mm chain gun and three 7.62mm machine guns, in a turret mount with advanced digital fire-control systems. It is highly mobile cross-country, much more so than the Bradley, and is also fully amphibious and air-droppable via medium transports like the C-130.
