Can I patent an invention if I discussed it publicly three months ago? How?

Yes, if you and/or you and your colleagues are the inventor(s) and were the first to disclose it publicly (3 mos ago). You can file a patent application provided you do so within 1 year for the date of public disclosure. After that, your disclosure will be prior art that may anticipate or make your invention obvious.

You have 1 year from when you talked about it publicly to file a patent application in the United States.

The answer is probably. In the US, you basically have a year after disclosure (other things can complicate this issue) to file a patent application. Also, it is likely that your talk did not enable your invention (depending on the complexity of your invention - some inventions are so simple that merely talking about it may fully enable it) and so you arguably can file your application. The safest way is to file an application before any public disclosure. I recommend speaking with a patent professional because details of your disclosure can change the answer.

You have one year from the time your invention becomes known to the public (and "public" can be just a few people in some cases) to submit an application to the USPTO. You've got approximately 9 months to get yours in.

The US Patent Law allows an inventor a one-year grace period from a public disclosure of his/her invention in which to file an application. However - anyone who heard your talk - could take your concept, modify it, and file before you - which might prevent your invention from being patented. The US law now rewards the "first inventor to file" with a patent - not the first inventor. Get your application on file as soon as possible. GOOD LUCK!