what is the code on your green card? If it starts with "c" then it is a conditional green card. Marriage based green cards are normally conditional. Conditional green cards have shorter expiration dates, two years I believe. If it has been two years since your green card was issued, you must have applied for a replacement.

This new green card generally has a 10 year expiration date. If you have already received this then you can do whatever you want. Your marriage is irrelevant.

As far as citizenship is concerned, yes you can apply after being married to a US citizen for three years but you can also apply after being a PR for five years. It will take two more years to get a citizenship. So I don't see how that's relevant unless you are planning to apply for government jobs(USPS, ...., Office of the President. ;) or work in classified projects.