Candlestick patterns
A harami or an engulfing pattern, confirmed by a third candle.
Three inside up starts with a large bearish candle, then a small bullish candle inside its body (a bullish harami), then a third candle that closes above the first candle's open. Three outside up is a bullish engulfing pattern followed by a third candle that closes higher still. The down versions are the mirror at a top.
The third candle is the point: it confirms the two-candle pattern instead of trading on it straight away.
For education only, not financial advice.