You want to dose 0,05 mg/L Fe to 75 L, this means 75 L * 0,05 mg/L = 3,75 mg Fe
This will be dosed with 3 mL* :
3,75 mg / 3 mL = 1,25 mg/ml, which is the concentration of Fe-DTPA in your stocking solution.
Your stocking solution is 250 ml, so you need 250 ml * 1,25 mg/ml = 312,5 mg Fe-DTPA...