Aluminum has three extra electrons and will easily let the Fluorine atoms use them. Because Aluminum has three, that means three Fluorines can bond. The name trifluoride means three Fluorine atoms are involved. You should know that even though there are two electrons involved in a bond, those electrons are used in different ways. Ionic bonds happen when electrons are given up by one atom to another.
Covalent bonds happen when electrons are shared by both atoms.
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