gistfile1.py

from collections import deque
last3 = deque(maxlen=3)

def pings():
  while True:
    yield timepassed() * 17000 # Assume timepassed is updated somehow

# This may seem not D.R.Y., but there are only three…
ping = next(pings) # ;) Who knows how the pings are gotten
print("Current average: " + ping) # avg = ping/1

last3.append(ping)
last3.append(next(pings))
print("Current average: " + sum(last3) / 2)

while True:
  last3.append(next(pings))
  print("Current average: " + sum(last3) / 3)