First, as indicated, identify that the market you are observing is in a bearish trend. Secondly, you want to locate a large bodied bearish candle that opened high but ended the period substantially lower. Then the next candle needs to open around the close of the previous large candle and close higher, but within the body of the previous. The third candle then has to be opened within the body of the previous but then close above the initial candlestick that started the pattern.
Candlestick patterns are a useful way to help locate trends that may reverse, and the Three Inside Up or Down is one of those. There are a few rules and the first one is that market must be in a bearish trend with the anticipation of a trend change because this pattern helps to indicate a potential change in the current market.