Sample size estimation for three armed clinical trial in R for comparing the Restricted Mean Survival Time

Question

I have a three armed clinical trial with two treatments and one placebo. I am to compare the Restricted Mean Survival Time among the three.

I need to find the sample sizes for each arms (equal allocation)

I know that R software has survRM2 package for calculating sample sizes for a RMST of two armed trial. The code as used is:

library(SSRMST) ssrmst(ac_rate=ac_rate, ac_period=ac_period, tot_time=tot_time, tau=tau, shape0=shape0, scale0=scale0, shape1=shape1, scale1=scale1, margin=margin, seed=seed)

So, my question is, how do I use this package to calculate sample size for a three armed trial (with equal allocation). How will the above code modify?

Any guidance will be very helpful.