I have read somewhere that Witr salah has to differ from the Maghrib Salah. So can I pray two rakhat and do Salam and after that do one more separate rakhat.
There are different opinions among the schools on how witr should be done. You can pray it like maghrib, or like maghrib but without sitting in the second rak`a for tashahhud, or you can pray two rak`at, say salam, then pray a single rak`a.
All of the above are acceptable ways of doing it unless you wish to strictly follow a particular school.
- Fatwa by Dr. Faysal Mawlawi of the European Council on Fatwa and Research (Arabic PDF)