Ammonia is a weak base, forming hydroxide ion by the hydrolysis reaction
NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)
Then the hydroxide ion combines with Fe3+ to form Fe(OH)3 (s). Overall, we have
Fe3+ (aq) + 3NH3 (aq) + 3H2O (l) --> Fe(OH)3 (s) + 3NH4+ (aq)
There ya go!
Steve