To calculate sample size for a three-arm randomized controlled trial (RCT), you will need to first determine the desired level of power, the effect size, and the number of arms. Then, use a sample size calculator to estimate the sample size for each arm. For example, if the desired power is 0.80, the effect size is 0.15, and the number of arms is three, then the sample size for each arm would be approximately 220.