How to compute gratuity in UAE?

I've been working for my company for 3 yrs and 7 mos, and my basic salary is AED 5000. I'm wondering how much gratuity i will get and how they would compute it. I've read one article that if I completed 3 yrs with my company, gratuity will be computed as 21 days wage. I'm confused...is it 21 days wage for the whole 3 yrs or 21 days wage per year? Please help. Thanks.