My wife she is legal resident in united state of America since 3 years ,the question is when she able to apply for citizen ship.

It depends on how she acquired her LPR status. She more likely than not has two more years to wait. Read the Guide to Naturalization published by the USCIS. It can be found at USCIS.gov

Dear Sir, in order for your wife to apply for naturalization under the 3 year rule of residency, she must be married to and living with you in a marital union for 3 years as a permanent resident. Regards.

Are you a US citizen? If so, she can file now. If not, she needs to wait 5 years.

Hi, If you are a US Citizen and you both have been married to each other for three years, then she is likely eligible to file for her Citizenship. USCIS has a worksheet to determine eligibility. If you require an attorney to assist you with this please contact my office for an Estimate and an in office consultation at 954-315-1160