I have been married to a US citizen for over 3 years and my green card was issued more than 5 years ago. On my application for US citizenship, should I file under being a lawful permanent resident for at least 5 years or should I use naturalization on the basis of marriage to a US citizen as my eligibilty? Thanks

If you are still married to the USC and that was the basis for your PR, you should file under the 3 year provision. You can file as early as 2 years 9 months.