Can we exchange our erc20 tokens in etherdelta for some valuable coins such as eth, btc, ltc
No, you can't.
Your question has answered above.
if you want to exchange erc20 tokens for eth, btc or ltc, you may try bittrex, poloniex, binance bitfinex or bithumb.
However, I would recommend to use a local exchange in your country as you may find it easier to verify your Id in case it's required to withdraw cryptocurrency, but in most case, no need to verify Id to withdraw cryptocurrencies, unless you want to withdraw fiat currency.