I have been working in Citigroup for almost 3 years. My supervisor told me there is a law says any employer with lay off rate more than 3% is not eliigible to sponsor a green card. Is that true? If that is true, what kind of law is it?

There are certain limit for foreign workers. Ratio depends on total staff, positions, etc. Also the employer has the right whether they would want to petition green card for some employees or not.