What is a ”three centre two electron bond”?
These are the type of bonds that are found in electron deficient molecules.
for example in case of BF3 , the molecule exists as a diamer that is B2H6 .
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.