Welcome to crypto!
There are several other exchanges out there beyond the big 3, I just haven't used them so I don't mention them yet.
Don't panic at these flash crashes. They've been happening across exchanges because of the exploding growth everyone is experiencing, Coinbase, Poloniex, Kraken. I don't think Bittrex has had a major crash yet, but it could happen any day as more and more people are looking for other exchanges.
Always have your wallets outside of the exchanges as well.
Good point!
33% exchange
33% mobile wallet
33% offline cold storage paper wallet