Randomization of treatments in R studio

I want to get a randomization of treatments with three levels and sample size n = 15. I'm stuck in where

volunteers <- 1:15 set.seed(1); sample(volunteers, size=5, replace=F)

I want three different groups, five each, but I'm new to R.

This is a data setup for ANOVA, not a specific question which gives particular data sets. Also I don't know what it means for set.seed