Here are the slides used in class.
View this video tutorial of how to write down molecular orbitals of BH3, and similarly, view this video tutorial of how to derive molecular orbitals for diborane, B2H6.
Please read this work on Photoelectron studies of boron compounds
Resources: (i) a story about Sheldon Shore, Small Boranes, and Ammonia, and (ii) diborane's calculated molecular orbitals in 3D for you to examine, and (iii) a slide set on B2H6 symmetry and molecular orbitals.