What is the formula of the compound in which the atom combining ratios are: boron : bromine = 1 : 3?

Boron with atomic number 5 belongs to Group-IIIA in periodic table. The electronic configuration of Boron is,

                                            1s², 2s², 2p¹

As there are 3 electrons in valence shell of Boron so it can make three covalent bonds.

Bromine with atomic number 35 belongs to Group-VIIA in periodic table. The electronic configuration of Bromine is,

                                1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p⁵

As there are 7 electrons in valence shell of Bromine so it can make single covalent bond to complete its octet.

Compound of Boron and Bromine:
                                                       One atom of Boron with capacity of forming three covalent bonds combines with three Bromine atoms with capacity of forming single bond to form Boron Tribromide (BBr₃).

Because it was a dramatically big guy