The 3 electrons of Al are covalently bonded with Florine to AlF3. However, the thing which is to be noticed that AlF3 has an empty orbital which can accomodate 2 more electrons. From the second AlF3 the Florine donates 2 pair of electrons to the empty orbital of first AlF3 and this done by the first one too, forming two dative covalent bonds. thus, forming ALF6.