1) At first you have to calculate the number of values of three periods.
3 * Period(Signal) / (Signal.X - Signal.X)
Then you can use the Signal Smoothing Analysis Object. The object calculates a smoothing average. This analysis object uses the Smooth- or the Mean-FPScript function to calculate the moving average.
2)FlexPro offers the index operator  or the value index operator []. These operators can extract sections from signals and signal series.