Those are some great pics and processing examples. I also appreciate the larger image scale.
Definite increases in contrast from single through triple stack, and unfortunately obvious decreases in image quality. I would attribute this to the increased length of exposures required. The Quark is not providing it's narrowest bandpass due to the lack of telecentric use, and it also has significant internal non-removable blocking and likely the least transmission of the three etalons in the system.
Judicious use of gain would be helpful to shorten the exposure time and degradation from seeing effects - at 60 mm you do not seem to have enough light to pull off triple stacking very well and need all the image brightness you can get.
Stacking etalons requires taking steps to maximize transmission. If you want to seriously pursue triple stacking further, here's what I would consider:
Get a second 60 mm non-richview tuned front etalon for triple stacking (Lunt 60, or original SM60). The SMII and SMIII richview etalon's ERF is integral to the filters operation and should not be removed as described below. (Note: the SMIII double stacked telescopes have both a richview and non-richview etalon - both of which are also tilt-tuned - but these do not seem to be currently available as separate items).
You can remove the RG630 ERF from this non-richview 60 mm etalon - your current SMII and internal etalon already have this ERF as well, so it's completely safe to remove them from the new front etalon for double or triple stacking use. This etalon with the removed ERF would be the second of the three etalons in series.
Alternatively, you could sell the SMII and get two non-richview 60 etalons. After removing both the ERF's, use a 75 mm Baader DERF ahead of both. This will require a fabricated DERF cell to fit the forwardmost 60 mm filter's case. It will take some finessing to optimize tilts to handle the retro-reflections between the etalons. This will optimize the front etalon double stack. The internal etalon/ERF will now be the third and final etalon in the system. You might also see if a circular polarizing filter could help if internal/external reflections prove problematic - but then again this filter significantly impacts image brightness.
Lastly, you'll now have to use the Lunt blocking filter (the Quark served as both an etalon and blocking filter). The Lunt BF's transmission can be improved by replacing the BG38 forward blue filter with a 1.25 inch thread-on KG3 UV/IR blocking filter.
Thanks again for your efforts and pictures.
Now when the weather improves I may have to give triple etalon stacking a try. Getting three of them to play well with each other is the main issue...