I lost my a tooth about 3 years ago. my teeth alignment is unaffected till now. is there any chance that my teeth may loose their alignment now?

Unlikely: Unless something else changes, your teeth should be stable in an adult mouth. Other medical conditions that cause bone loss, jaw or tooth trauma, or mouth soft tissue infections could in theory cause teeth to shift but this would be somewhat unlikely (and still a very slow process).

Drifting: A lot depends upon which tooth was lost, your occlusion and its stability and your periodontal health. Even though there has been no drifting for three years (according to your evaluation or your dentist's?) as you get older there is more likely drifting to occur than not, although some people never have teeth drift even over decades.

Yes: Teeth continue to move throughout life. Not replacing an extracted posterior tooth is usually a template for posterior bite collapse. If an anterior tooth is lost expect eventual tipping and drifting. See your Dentist to evaluate the stability of your dentition and the treatment alternatives that are available. Unfortunately many people wait too long because movement is not obvious early-on.