What is a ”three centre two electron bond”?
These are the type of covalent bonds in which overlapping of orbitals of three atoms take place but sharing only two electrons.
These type of bonds that are found in electron deficient molecules.
for example in case of BF3 , the molecule exists as a dimer that is B2F6 .
Here the boron with atomic number 5 has 3 electrons in the valence shell and by making a covalent it can make it upto 6 only . so to make it happen or to have 8 electrons in valence shell Boron makes this bond.
this 3 centred and 2 electron is a best example of octet completion of electron deficient molecules or species.