This profile should get you close.

Here is the profile for Sunlu PETG, Prusa prints higher, so you may need to adjust the temp. Print a temp tower.

It took me a long time to get PETG perfect, still sticks more reliable on the MK3S then the C1L.

I also have a Lack enclosure, that helps. PETG likes staying warm with no draft.

The Live z is critical to PETG on the MK3. You need to get this perfect. https://forum.prusa3d.com/forum/postid/786532/

Cheerio,

I have issue with PETG only, well dry, printed or I will rather say trying to print PETG in a Prusa Enclosure heated at 30/35°C.
Despite having check my bed leveling which seems to be quite good, with a Live Z Offset at -0.38 now , I am still fighting with the 1st layer and even more from the start of the small oultine not sticking/adhering to the steelsheet from and for the very first point, and I don't know what I missed.
Happy to get your insights or suggestions ;)

Printer : MK3S+ 
Nozzle 0.4
Print settings : back to the basic ones in system presets : 0.20mm Quality (and try speed too first, also tried the 0.30mm draft). 
steelsheet : Textured
filament Prusament PETG 
mainly took the set from The Perfect Prusa i3 PETG Settings
Nozzle temp : 230 then 240
heatbed temp : 85 then 90
cooling fan speed min 30% max 50%  Auto cooling : ticked ON , Keep always FAN ON : unticked
Bridges fan speed 50%
disable fan for the first 3 layers
full fan speed at layer 4
brim width 5mm
Infill 15%
travelling speed : 130mm/s
Retractation speed 40mm/s
Retraction length 5mm
1st Layer speed : 20mm/s
speed for print moves in mm/s: 
   Perimeters : 45
   small perimeters : 25
   external perimeters : 25
   infill : 80
   solid infill 80
   top solid infill : 40

Why do I miss ? 😭 

- Core One L
- textured sheet
- Prusa ObXidian 0.4 nozzle
- PCTG from nobufil
- Creality SpacePi X4 dryer and I am printing out of the dryer and it is switched on during the print - drying modus or "Auto"-modus with 20% threshold

- starting preset was the "Spectrum PCTG" preset in Prusa Slicer
- final tuned nozzle temperature: 256 °C
- final bed temperature: 90 °C (from the beginning)
- fan speed changed to nobufil settings - 30% up to 50%

- print mostly with "0,15 layer Balanced" (one of the presets in easyprint)
- larger parts "0,15 layer Structured (for a perfect surface and better layer bonding)

- first print was the Benchy from Prusa on Printables (the Benchy on the USB stick ist heavely optimized for PLA with factory out Standard HF nozzle)
- observed the result when the first layer was printed
- adjusted nozzle temperature
- print finished flawless
- never touched "z-axis" settings
- never printed a calibration model

My conlusions/recommendations:
- start with a preset or the closest preset when not using the preset filament
- observe the first layer(s)
- tune settings during print
- dry, dry, dry, dry your filament or keep it dry at least
(- printing policy when using a filament the first time: First print is the Benchy for tuning 🙂 )

The Filament settings, I had a decent profile of my own based on the default PETG , but was still not happy. Since Prusa makes parts out of their own PETG for thier printers,  knew it could be done, but the default profile wasn't it.  I grabbed one of the Prusa Mk3 files from thier Core one + printable parts files and looked it, found a bunch of changes that are not in the included default Prusa profile. Lots of testing after that, problem solving for bed adhesion, warping issue, chamber fans.......

It is just a combination of all things, print settings, extruder tuning, z offset, filament settings, temp tower, bridge test, stringing test, elephant foots test, chamber fans, print sheet.  Get it all right and you get a good repeatable result.

Each time I start a new filament, I go through all the steps again. The better base profiles I build,  the quicker it becomes to add more of that type. I may spend an entire day + just testing a filamanet, sometimes much longer, like the PETG.

Posted by: @maxluzi

same for me. i´ve been printing with my mk3s (with enclosure) a few spools of cheap pla and petg perfect with the generic profile.
i also printet really a LOT ASA, PLA and PETG with my mk3s almost without any issues. It was running for about 4 years until i bougth the COL.
i have never needed to change some fillament seetings, except first layer temp or something like this. so there was just no need to get any deeper in into this.
now with the new printer i should make a calibration for every original prusament? c´mon. this was far away from a little tuning. the prints went from worse to good wit  your petg profile.. 
if you @ruebarb could tell us the changes you made in the filamet it will save me or us some study. i want to understand what was wrong...

Looking at it I see there aren't that many differences, maybe the original profiles were corrupted?

With few exceptions, I print only budget material and found the "generic" stock presets are spot-on. A little bit of tweaking here and there to correct for actual shrinkage (XY and Z multiplier), extrusion multiplier for fine tuning, possibly raise temperature but nothing that should cause functional failure as in your pictures. It's not that sensitive - or if it is, this points to some deeper, underlying issue.

This statement holds for several 2nd-tier brands of ASA, ABS, PETG and unicorn-pink PLA which I only touch with rubber gloves.