Strength training: is it better to do three intense days or five light days?

It's not really a matter of choosing between two fixed options - it depends more on how much time you can dedicate to it and how you recover. Three intense days work well if you can really push hard and get enough rest between sessions, while five lighter days let you train more frequently but you need to be careful not to overload your joints. Honestly, what matters most is being consistent over time and actually sticking to the program without skipping sessions, rather than what the exact number of days is.