Scrum of scrums for three teams and its duration

Having an event where teams can share their dependencies help, not necessary you have to do it while standing :-)

If you have 3 teams working on the same product, might the Nexus Framework be appropriate in this case?

Do we need SOS for three teams

That is a question an internet forum cannot answer for you. Do you need a meeting to synchronise your teams? That is to say: Are there any dependencies between them? Are there any other benefits from your teams sharing their plans to achieve the sprint goal?

what should be duration for this meeting

As long as needed and absolutely no longer than necessary. There are a few different ideas floating out there that are called "Scrum of Scrums" with different participants, different frequency and hence, different timeboxes. Find out what it is you want, then figure out how long it takes. Then try to make it shorter ;-)

Also is SOS a standup meeting?

I feel it's a bit micro-managy to tell people whether to sit or stand during a meeting. It usually takes the participants about five seconds to decide whether to hold a meeting sitting down or standing up. ;-)

Thanks Sanjay Ian and Julian for your comments.

@Ian, yes I agree with  you, have to go through this.