I have had an anterior hip replacement 3 weeks. ago . after a days activity is it usual to feel a lot of discomfort in the groin area?

Yes: Not unusual to have aches and pain for 6-12 weeks in general. Most joint replacements improve over 12-18 months when they usually achieve their full recovered state. You still need to make your follow up appointments so your surgeon can assess you. Any worsening pains, fever , chills you need to call your surgeon with right away. But groin pain after activity is your body saying take it easy.