The Chicago Tribune article references the following infographic from The Land Connection: https://www.thelandconnection.org/wp-content/uploads/2019/03/local_food_seasonal_calendar-health-benefits1.jpg
I've cropped the relevant section, here:
The quoted "Most produce loses 30 percent of nutrients three days after harvest" seems to refer to the text towards the bottom of that section, "Day 3. A majority of fresh vegetables have lost at least 1/3 of their nutrients."
The infographic has two sources listed in this section; Rich Pirog et al "Food, Fuel and Freeways" and IFR Extra Ltd. The Food, Fuel and Freeways paper is solely about food miles, and makes no mention of nutritional value. I've found mention of the IFR research quoted in several newspaper articles; it ultimately seems to stem back to a study they were commissioned to undertake by Birds Eye, the frozen food producers, to see if an earlier study, "A comparison of the vitamin C content of fresh and frozen vegetables"
by D.J.Favell, (published in 1998 but written in 1996) was still relevant.
Looking at that study we can finally answer the question; the study was from the mid 90s, the only nutrient tested for was vitamin C, and the only vegetables tested were peas, UK; green
beans, Italy; spinach, Germany; broccoli, UK; and carrots, Austria. Looking at the graphs from the study:
We can see that the roughly 1/3 loss of vitamin C after three days seems accurate for the peas, beans and broccoli stored at ambient room temperature, though not for the chilled vegetables or the carrots. The vitamin C in the spinach did drop off a lot more sharply.