Order of evaluation in piped expressions

I don't understand in which order piped expressions are evaluated.

Consider the following example:

f1 <- function(x) {
  message("called f1")
  paste0(x, "_f1")
}

f2 <- function(x) {
  message("called f2")
  paste0(x, "_f2")
}

f3 <- function(x) {
  message("called f3")
  paste0(x, "_f3")
}

"a" %>% f1() %>% f2() %>% f3()

which prints:

called f3
called f2
called f1

and returns:

[1] "a_f1_f2_f3"

Can you please explain exactly how the expression is evaluated?

Which function is called first, which is called second, which is called last?