The Japanese have a lot of military analogies for Candles and Candlestick patterns. Three Advancing White Soldiers is another one. Others would be Piercing Patterns, Hanging Man, and Gravestone Doji.

Japanese traders made Bullish Candles white, hence the term "White Soldiers". I prefer using green for a Bullish Candle and red for a Bearish Candle. Of course, you can use any colors you want.

The three white soldiers are a slow and steady rise over three days. Each following candle should open somewhere high within the previous Candle. The following two Candles should never close lower than the prior candle. In other words, there should be higher highs than in the previous two Candles or the pattern is not valid.

This pattern should happen towards the bottom of a trend. It does not have to be the exact bottom to work. It may show up a few days later and still be a valid pattern.

Something I feel that can make this pattern even stronger is if the first Soldier (Candle) happens to be an engulfing pattern as shown below.