Just for clarity, say I use Tawnos's Coffin to exile either a Triskelion or Tetravus and it has three +1/+1 counters on it. When I untap the Coffin and the exiled creature re-enters the battlefield, it will enter with the previous counters plus an additional three for ETB effect?
In general, an effect that says a permanent "enters the battlefield ... with" a given number of counters (C.R. 614.1c), or that says to "return" a card "to the battlefield ... with" a given number of counters, adds that many counters to that permanent as it enters the battlefield, whether the effect uses the word "additional" or not. (See also this thread and this thread.) Thus, for example, in this scenario, Triskelion or Tetravus will enter not only with the "noted number and kind of counters" for Tawnos's Coffin on it, but also with an additional three +1/+1 counters on it.
Tawnos's Coffin with Triskelion/Tetravus