I have not yet written MAT, I am having 3 years of work experience (still counting).

Ok. Incase you have not given MAT then try to score above 600-650 to get into top management institutes . 3 years of work exprience is considerable. Few of the colleges are:
1. ASM institute of Professional Studies
2. BIMM
3. IES Management College
4. Atharva institute of Management
5. Institute of International Business and research
Check their official site as well. Good Luck.

In mat may 2017 my composite score is 628.5, what are the college I am eligible for? can I get a merit seat from my marks, my interest is to MBA in OR.. please suggest.

You have a good score.

You can check your chances through MAT pathfinder tool.

http://www.bschool.careers360.com/pathfinder/mat