library(data.table)
library(stringr)
library(pipeR)

# Three rows as a group
nrowGrp <- 3

res <- data.table(matrix(c("S11","R1","O11",
                            "S11","R2","O12",
                            "O11","R3","O12",
                            "S21","R1","O21",
                            "S21","R2","O22",
                            "O21","R3","O22",
                            "S11","R1","O11",
                            "S11","R2","O12",
                            "O11","R3","O12"),
                          ncol = 3, byrow = T)) %>>%
    # use pipeR to storage number of columns
    (~ nCols = ncol(.)) %>%
    # add group column
    .[, group_num := ceiling(.I/nrowGrp)] %>%
    # by each group join elements to one string
    .[, .(grp_val = t(.SD) %>% paste0(collapse = ",")), by = .(group_num)] %>%
    # summarise each group count
    .[, .(group_count = .N), by = .(grp_val)] %>%
    # separate string to multiple columns
    .[, paste0("X_", 1:(nCols*nrowGrp)) := data.table(str_split_fixed(grp_val, ",", nCols*nrowGrp))] %>%
    # remove grp_val column
    .[, grp_val := NULL]