Here from Google search I guessed that the address that starts with 3 is known as segwit address .
No. Addresses starting with 3 is a P2SH address. They can be a non-segwit address too.
If its a Segwit, it would be a P2SH-P2PWKH address.
And second thing if I am right then give me any site ( trusted 100% ) from where I can get my segwit address .
Electrum.org or
https://bitcoin.org/en/download.
And also not only I want get address but also can import in mycelium Android wallet using private key .
MyCelium does not support Segwit. In fact, none of the Android wallets which I currently use support segwit.
Greenaddress for Android should support segwit, if I remember correctly. I don't really like them though, lost quite some BTC a few years ago.