Congrats on the new cards.
As others have noted, ensure you meet the spend requirement to get the BCE bonus.
Check the Discover site, because you have to activate the Quarterly Category bonus. The upcoming Q3 Discover is Dining, which means you want to use Discover for any Dining costs in Q3, getting the 5% and building up for the 2x at the end of the year. Always activate the Discover category leading into each quarter.
Look at your American Express on-line account, and check for any Offers below the main page. These can lead to various spend bonus items. You have to activate and make special steps to ensure you qualify the spend for the bonus, but they can be quite lucrative for the spending you would do anyway.
Use all three cards. Discover will work many places, but not everywhere. AMEX will work a few less places than Discover, but can be useful for the bonus / specials. The Capital One card should be in your wallet always, because there will come the day that neither the Discover nor the AMEX are accepted at the merchant / restaurant and having the Capital One will get you through.
Use all three cards 
