Well, there are 6 electrons to distribute in ##BH_3##, however, ##BH_3## does not follow the pattern of ##”2-centre, 2 electron”## bonds.
Boron has 3 valence electrons, and hydrogen has the 1; thus there are 4 valence electrons. The actual structure of borane is as diborane ##B_2H_6##, i.e. ##{H_2B}_2(mu_2-H)_2##, in which there are ##”3-centre, 2 electron”## bonds, bridging hydrogens which bind to 2 boron centres. I would suggest that you get your text, and read in detail how such a bonding scheme operates.
By contrast, in ethane, ##C_2H_6##, there are enough electrons to form ##7xx”2-centre, 2 electron”## bonds, i.e. the ##C-C## linkage, and ##6xx”C-H”## bonds.