The 3D pattern is Burton's older proprietary mounting system that uses a triangular three-hole arrangement instead of the standard four-hole grid. It was common on Burton boards for years but is now being phased out in favor of their Channel system.
The catch is compatibility: 3D boards only work directly with Burton 3D-specific bindings. If you have non-Burton bindings or newer Burton bindings, you'll need an adapter disc to make them fit. Most binding brands offer these adapters, but it's an extra hassle and cost.
Since 3D is an older system, you'll mostly encounter it on used or older-model Burton boards. It also gives you less stance adjustability than modern 4x4 or 2x4 patterns, since the spacing increments are wider. If you're buying new, you likely won't see 3D—but if you're shopping the used market, always check the insert pattern before buying.