When you sell stocks in a cash account there is a 3 day period before the payment settles and you can use the money to buy something else. In a margin account there is no waiting period on the money so you can use the money from a sale to buy something else right away. As for the 3 rountrip trades, this is a fed rule for pattern day-trading. If you buy/sell or sell/buy the same security within the same day it is a day-trade. If this is done 3 times in a 5 day period you are labled as a patern day-trader and are required to maintain $25000 minimum in a margin account.
Hope all this helps. Your brokerage firm should have all of this info in more detail.
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