Metal fluoride thermal stability trends are largely dictated by lattice enthalpies; what matters is the relative size of the cation in consideration (the charge/radius ratio - a measure of polarizability - is also a key factor). Smaller cations can better hold small anions like fluoride. The larger the lattice enthalpy factor, the tougher thermal decomposition becomes. As atomic size decreases across a period, the order of stability is AlF3 > MgF2> NaF.
picture them in your mind
4.50/2.00 = 3.00/n²