Is BH3 Polar or Nonpolar?

Answer: BH3 is a nonpolar molecule due to the symmetrical trigonal arrangement of the hydrogen atoms around the central boron atom. Furthermore, the low electronegativity difference between boron (2.04) and hydrogen (2.20) means that the molecule contains no polar covalent bonds, thereby negating the possibility of any permanent partial dipoles within the structure. 

BH3, also known as Borane, is primarily characterized by the presence of an incomplete octet. This means that the central Boron atom does not have a complete octet of valence of electrons. As a result the molecule has a high affinity for electrons and is a strong acid. Due to these properties the molecule is highly unstable and reactive although it can be observed for brief periods of time in nature. The structure appears as a transition molecule in certain specific longer reactions; however it does not appear stably in a pure form in nature.

BH3 Ball and Stick Model. Created with MolView.

Have other forms of this molecule also been observed?

Although this form is not the exact same molecule as BH3, BH5 is a closely related species of borane. Instead of their being an incomplete octet, BH5 has an expanded octet which is traditionally considered to be impossible for elements in the first three rows of the periodic table. Therefore it makes sense that this molecule has only been observed stably at very low temperatures since it is incredibly unstable and even the slightest energy input from the atmosphere is enough to destroy it.