The M113A3 can include various weapon systems including a ring-mount or a remotely operated weapon station armed with a 7.62mm M240 machine gun, a 12.7mm M2HB heavy machine gun, or a 40mm Mk.19 automatic grenade launcher. Additionally, up to four soldiers can engage in combat from the cargo hatch, often supported by shielded machine guns. This machine gun can be used for close defense against lightly armored targets and enemy troops. It provides additional firepower to support the troops transported inside the vehicle.
In addition to its turret-mounted machine gun, the M113A3 armored vehicle can be equipped with various other weapons based on operational requirements. These include light 7.62mm caliber machine guns mounted on gun-shielded mounts on the cargo hatch, allowing up to four soldiers to engage in combat from within the vehicle. Furthermore, the available space on the vehicle's roof allows for the installation of additional weapons such as light anti-tank guided missile launchers, 90mm recoilless rifles, and 60mm mortars. However, it's important to note that these additional weapons are not standard equipment and are tailored to specific mission needs. 

The vehicle's design is similar to the previous version of the M113, and it can accommodate up to 11 military personnel, including a driver and a commander/gunner. At the rear of the vehicle, ten fully-equipped infantrymen are seated on side benches, arranged in a configuration of five individuals facing each other. The driver occupies the front left position, with the engine located to the right, while the vehicle's commander/gunner sits at a central location, overseeing operations and coordinating the movements of the onboard team. 
The M113A3 has a box-like appearance with a well-sloped glacis plate and an inwardly-sloped rear. It features a large trim vane on top of the glacis plate, pioneer tools stored above it, and pairs of headlights with collision guards on each side. Smoke grenade dischargers are often mounted on the upper glacis plate. The rear end of the vehicle is characterized by a ramp with a hinged door on the left side, flanked by large fuel cells. The roof is equipped with various fixtures, including hitches for attaching additional armor, a cargo hatch, and a commander's cupola with a machine gun mount. The front of the roof houses the driver's hatch, engine ventilation, and exhaust louvers.
The armor of the M113A3 remains unchanged from previous models, using 5083 aluminum alloy. It is 44 mm thick over the frontal arc, 38 mm thick over the sides, rear, and roof, and 28 mm thick over the belly. While this armor provides protection against anti-personnel mines, small arms fire, shell splinters, and blast overpressure, it is vulnerable to vehicle-fired anti-tank munitions and continuous heavy machine gun fire. Spall liners and a collective NBC (nuclear, biological, chemical) protection system are standard on all A3 variants. Optional appliqué armor kits for protection against larger rounds were offered but not widely fielded by the US military. Some M113A3s operating in Iraq have been fitted with bar armor cages and 25 mm ballistic steel plates as interim measures against shaped charge warheads.
To enhance protection, spall liners and a collective NBC (Nuclear, Biological, Chemical) protection system are standard on all A3 variants. Additional armor kits for protection against larger calibers were offered by BAe, but they were not widely adopted by the US military. Some M113A3s in service in Iraq were fitted with bar armor cages and 25 mm ballistic steel plates as an interim measure against shaped charge warheads.