Problem Description: After the car has been heat soaked in a parking lot, and you go out to it the HVAC will be running full bore with recirculate on and there will be a "dirty-feet", vinegar or generally stale smell. It lasts about thirty seconds to one minute and then it is gone. This issue is described all over the different forums for Model 3s and apparently there have been (a) firmware fixe(s) that have been pushed out to deal with it. If you do not see water collecting under your car you may have a condensate drain blockage causing your problem and you will probably need Tesla Service's help getting that fixed. But, if you do have water collecting under the car that is good and this write up may address your issue. I tried using the Mothers car deodorizers and that just masked the smell (a little) and did not solve the problem. Usually i1Teslas videos are super helpful but this one did not solve my problem: https://youtu.be/KOYyYzPl5e4. If your issue is general pet smell and maybe a little different than mine that video may help you so I have included it.
Difficulty: If you can change the oil in a car without making a total mess of it you should be able to do this.
(In the steps below I refer to the part that needs cleaning as the condenser @vlinev pointed out that this is more correctly referred to as the evaporator. Apologies to all those who are HVAC engineers. In case it aint obvious I just play one here on this forum ;) )
1) Remove and clean both your air filters. These are my air filters, note the 2017 date of manufacture: https://imgur.com/xptPU5p This will not solve your issue in any way but now that you have them out you will be able to inspect and clean them, which is a good thing. Follow this official howto from Tesla https://www.tesla.com/support/do-it-yourself-model-3#replace-air-filter and/or this video on youtube https://www.youtube.com/watch?v=WCteH3AxVQo to get the filters out. You need to take them out to get to the condenser to spray it with the Klima cleaner.
2) Once you have everything out, the condenser looks like this: https://imgur.com/mBX9PGp
3) Replace the Klima cleaner nozzle with the long nozzle provided with the purchased Klima cleaner. At this point I put down a trash bag to generally cover the footwell area in case there was overspray. But I did not have any.