50 mL 3.0 M HCl(aq) that means hw much HCl i have to add
HCl you need? HCl is a liquid, what is your stock solution concentration?
Assuming the stock solution is 12M, then you need to dilute it 4 times, or three parts water, one part stock solution.
one part of 50ml is 12.5ml HCl stock solution, then 3 parts (37.5ml) water. Add the acid to the water.