Plants need exceptionally low levels of Manganese.
What you are seeing is an immobile element deficiency.
Discerning Mobile from Immobile is usually very easy. Blotchy and non-uniform spotting located between veins and on veins indicates immobile element deficiency. Mobile element deficiency is usually more uniform and located between veins.
This is a result of how elements move in the plant. Nothing fancy.
The primary immobile element that would cause this issue is Calcium. Phosphorus and Calcium bind for mobility and are interrelated when it comes to utilization, which might help explain some of the issues (if your buds aren't forming as large as you'd expect).
Being that you are 3 weeks from harvesting, I don't think adding more of anything is going to resolve the issue. Also, Calcium is not in high demand during flowering. If the issue you have was related to salt toxicity (namely Potassium) you would be seeing issues with Magnesium as well. The yellowing of the leaf would actually reinforce this diagnosis. The lack of necrosis though seems to indicate otherwise.
Since the spotting looks fairly old, the overall look of the plant still mostly healthy, what I suggest you focus on as a solution (despite the "correct" diagnosis) is to rinse the media with pH balanced water and top dress with 1 Tbsp dolomite lime per gallon of soil and water through it. This will establish a pH buffer, correct any pH deviance in the media, as well as provide a complete micro-nutrient profile for the plant to utilize.
When it comes to spots and yellow leaves 3 weeks from harvest the last thing you want to be doing is mistreating the issue. Fortunately, the solution for nearly EVERY issue in a media grow is to simply flush the media and establish a healthy buffer of nutrition once again. At some point one of your ratio's went out of whak. Regardless of whether it is toxicity, or pH, or anything else, the focus here should be on the media and the solution used. Not on the plant. When you react to issues on the plant you are already knee deep as it takes time for these issues to show. Often times in flowering "windows of opportunity" open to push the plant with specific nutrients. Keep a journal of when this issue started so that the next time you grow this strain you can anticipate the need for micro-elements and a balanced pH.
Of all the things you could purchase to help resolve the issue a reliable digital pH meter should be first on your list. They are only $40... and when it comes to not destroying your plants... $40 well spent IMO.