At 3 weeks you could be just starting feeding, but even half strength is way too strong. You need to be down like 1/6 strength, or 1/4 strength. You want to be in the 200-300 ppm range, including what's in your water. So if you started feeding a week ago, you def burned them, and that's the source.
So when you "burn" a seedling, what you're burning is root growth. What you're seeing on the leaves is just a sign of what happened below a little earlier. Stick with plain ph'd water for another week or two so this plant can try to recover. It has to start pushing out new healthy roots that not only support the size it is now but will allow for growth. When you start seeing new, healthy growth for a week or so, then you can start feeding again, but seriously, keep it to about 1/6 strength for a bit if you don't have a ppm meter. Then after another couple of weeks, go to 1/4 strength. I've run entire grows at 800 ppm max, which is about 1/3 of what the label on the nutes bottle says.