Observer is normally supposed to detect updates instantly, but instead can take up to 3 ticks to update. (same amount as a repeater set to 3 tick delay). This leads to timing inaccuracies in double and triple piston extenders. This is not a consistent delay, and causes these circuits to behave differently each time they are powered on and off.
The standard problem with triple piston extenders is getting it to power off. I have managed to get this system functioning flawlessly.
The first attached image is the power-on component of the TPE. The circled (3 tick) repeater fires before the (2 tick+observer) repeater to the right.
The second attached image is the functioning power-off component of the TPE. No problems here. Resets same way from fully extended position each time it is powered off.