Calculating Power and Choosing Analysis for Pre/Post Intervention

The study I have designed will measure medical adherence, depression, and anxiety via three different measures pre and post intervention. I am thinking of analyzing each measure separately via dependent t-tests. I have two questions: 1) Is there any benefit to analyzing via repeated measures ANOVA? 2) What is the best way to calculate the number of participants needed? At this point I used GPower with t-tests, "difference between two dependent means" and a priori with standard tails/effect size/alpha/Power settings. Should I be considering the fact that there are three different variables in calculating power? I feel like I should be but don't know how via GPower.