Hi Nazar,
I think you want to write to your management to consider an increment upon completion of your 3 years service.
In case your organization does not have a system of Appraisals, Increments, Annual Performance Review, etc.... then you can write to your management to increase your salary.
Your application should contain much about your performance during the last 3 years, specific achievements, contribution to the organization, the responsibilities that you are handling...... so on.
Hope, this is in line with your requirement.
Ragards,
Ramesh Karasi.