Hi,
The three day wait will go away once you sell your stock the funds will be ready to use again no intrest will apply.
Remember if your account is under $25,000 you have to be carefull not to trade more than 3 DAY TRADES in a 5 day period. So if you buy x,y,and z today you can sell x,y, and z the next trading day. If you buy and sell x,y, and z all in the same day you can no longer day trade for another 5 days. If you break this rule they can suspend you margin account from you. Thats how it was with scottrader but i think that is how it is for all trading sites. It's a gov. rule.