Hello aspirant

hope you are doing great. As per your query  I would glad to tell you that generally duration of diploma in graphic design is around 1 year and diploma in advanced graphic design is around 4 to 7 months . I would advise you to go for 1 years because 3 months only you will need to clear the fundamental of yours in design and if you are looking for short term courses then you can follow udemy to save time

Hope you found this answer helpful. Good luck for your future