There's another reason why Al(NO3)3 has the value of pH < 7 in its solution. You will think the reason of it is Al3+ will be hydrolyzed and release H+ ions. No, that is not the real reason that makes a lower pH of Al(NO3)3 solution. Al3+ will form a complex ion with water first in its solution [Al(H2O)6]3+. These ligand that make a lower pH. See the figure as follows as the illustration:
Especially, the presence of the hydronium ions that make a lower pH of the solution. Now can you explain which cation has the lowest pH in its solution: Co2+ or Fe3+ ?
Mostly, the coordination number of a complex ion is 2 times the oxidation number of the cation. So that we can assume that the only species that exist in Co2+ solution is [Co(H2O)4]2+ and [Fe(H2O)6]3+ for Fe3+ ion. Because the amount of H2O of Fe3+ is greater than Co2+, it makes the presence of hydronium ions of Fe3+ greater than Co2+ too so that the pH value of Fe3+ < Co2+.