"If you have a Bitcoin (BTC) balance on Bittrex during the BCC UAHF time on August 1st, 5:20am (12:20pm UTC), you will be credited the equivalent amount of Bitcoin Cash (BCC) on a 1:1 basis. i.e. 1 BTC on Bittrex held during the on-exchange snapshot will get you 1 BCC "- states Bittrex on the webpage.
The Poloniex team is not sure if they will support Bitcoin Cash. "Even if two viable blockchains emerge, we may or may not support both and will make such a decision only after we are satisfied that we can safely support either blockchain in an enterprise environment. If we decide to support and make available derivative blockchain tokens after a blockchain split, note that if you have your tokens on loan at the time of a split, we cannot distribute the derivative tokens to you, the lender, as you would not possess them. Such loaned tokens can, and often do, leave the platform. " - said the Poloniex team in the recent update. They suggest you to withdraw your Bitcoin if you don't want to leave the decision to them.
You can find information about Coinbase support of Bitcoin Cash here.