I know a wallet is a private key can generate more than 1 address right?
Every wallet address has a single private key same as mentioned above.
how can blockchain.info support more than one coin (Bitcoin, Ether and bitcoin cash ) using one private key?
Its impossible that one private key has more than 1 coin, as mentioned every wallet address have 1 single pivate key even its ETH, BTC or BCH.
if not how can export other private keys? I found one private key only.
There are many tutorials on how you can export private keys of blockchain using its seed only the BTC, I dont know how to export the private keys of BCH since I never visit its new wallet with BCH but you can get the private keys of ETH directly using the desktop browser.
also can a wallet support more than 1 private key for bitcoin {2 or more private key}?
Obviously yes, HD wallets like blockchain.info have more than 1 private keys since it has more than 1 wallet address For BTC.a