Do you mean rung 3 of the very first ladder? I would not expect somebody's heart rate to spike that high that early, unless the bell is just under 5 RM weight. Might be a work capacity issue. If you say that you needed to slow down your training due to elbow pain, and it means you had to train at a slower pace with a high intensity bell, then your work capacity might have suffered a bit. Find a comfortable rest time between rungs and progressively shorten it.