What you are describing is called a wash sale. You can sell a stock and immediately buy it back (or within 30 days) but any losses cannot be recognized. Instead the loss is added to the basis of the stock when you buy it back which will reduce your taxable gain in the future when you sell it.
I believe the three day thing you are talking about pertains to how many brokerage accounts require three days from the sale of a security for the money to clear. Until then you cannot use the money gained from a sale to purchase new securities.
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