so there is a 3% fee on BTC deposits? So once I generate a BTC address I can always use the same one? (i.e. I can add it to my address book?)
Yes - the BTC address stays the same, so you can add it to your address book.
why there is a deposit 3% fee? I can't understand that and the fee is too high, and if i deposit dogecoin there, can i also get 200% deposit bonus? If i can, i just deposit dogecoin and can avoid to pay 3% deposit fee.
The fee's there to prevent people from using us as a free exchange - of course you can deposit dogecoin and avoid the fee! Your first deposit, whatever method you use, will receive the deposit bonus.