Can anyone advise me as to the three biggest exchanges? Only for Bitcoin / US Dollar (BTC/USD) exchange.
At this stage, I am not interested in any of the Alt currencies though I may be interested in Ethereum later.
Top 3 'the biggest exchanger' based on vol for BTC/USD and Vice Versa in my view for now:
Bitfinex,
Gdax,
Bitstamp,
you can go here for more information about it and control it everyday because it can change at any time
https://coinmarketcap.com/currencies/bitcoin/#marketsi don't list Poloniex because it's not really safe to use and many people have a problem in there,
so try to avoid this kind of exchanger for now because trading is a high risk activity and doing it in a shady exchanger make it even worst.
but the better exchanger in my opinion for doing a daily trade is Kraken,
because i never heard they have an attack from hacker just like Bitfinex and Bitstamp