PROC POWER, three treatments, binary outcome

[AndrewZ]

A non-profit wants to know whether sending a communication improves constituent retention, and is it worth sending an expensive print piece over a free email?  The communication can be sent by email, snail mail, or not at all (i.e., three treatments).  The expected retention rate after a year in the untreated group is 93%.  Any measurable difference in retention  (0.5 percentage points or greater) would justify the costs of the snail mail.

How would I calculate the sample size?

If there were only two treatments (snail mail and no mail), I would do this:

/* Is the improvement at least 1 percentage point? */
proc power;   
    twosamplefreq groupproportions=(0.93 0.940) test=pchi power=0.80 npergroup=. sides=1; /*  npergroup=7514 */
quit;

Should I use LOGISTIC?

In R it seems I would want pwr.chisq.test.