library(glue)

participants <- c(1:30)
tasks <-  c("a", "b", "c")
sessions <- c(1:20)

# Loop through participants, task, sessions
for (id in participants) {
  for (task in tasks) {
    for (session in sessions) {

      # Set an individual filename
      file_name <- file(glue("P{id}_T{task}_S{session}.txt"))

      # Define the content of the text file
      this_text <- "Your text here"

      # Write to file
      writeLines(this_text, file_name)

      # Close file
      close(file_name)
    }
  }
}