3-bolt refers to the hole pattern on the bottom of the shoe that your cleat bolts onto. It's the standard for road cycling and works with the major road pedal systems: Shimano SPD-SL, LOOK Keo, and Speedplay (with an adapter plate).
The main benefit of 3-bolt road cleats is their larger contact area with the pedal, giving you a rock-solid platform and efficient power transfer when you're putting down watts. That's why road riders, triathletes, and time trialists almost always choose this system.
The trade-off is that 3-bolt cleats protrude from the shoe sole, making walking clunky and slippery. If you plan to walk around at coffee stops or need a shoe for commuting with off-bike time, consider 2-bolt (SPD) shoes instead. But for pure road riding performance, 3-bolt is the way to go.