There is only one file type that uses the .L3B extension:
LEGO 3D Model File
An L3B file is a 3D LEGO file created by SR 3D Builder, a program used to build digital 3D LEGO models. It contains the dimensions of a 3D LEGO model which include their placements within a 3D coordinate system. L3B files can be used to store a variety of LEGO models such as dune buggies, trucks, and front end loaders.
While the L3B file is used to store LEGO models, you cannot print the model using the L3B file. However, you can open the L3B file with the LDview program and export it to the .STL format, which can be used for printing.
Updated: February 19, 2016