I don't know what your goals are, but: getting slim is 95% diet... Sourcing calories from diet is almost always cheaper than on a treadmill. Seriously. Generally slow and steady is the way to get it done, too much too fast makes rebounds. Low-impact exercise with constant wear is also a lot for joints...
Lifting heavy weights is much better at burning fat than cardio. It builds fat-burning muscle, and increases your fat burning across every hour for a long period post-workout. It's hormonally optimal with avout 45 minutes of effort. 3 x 45min would save you over 5 HOURS of gym time every single week. You could learn Chinese in half a year with that kind of investment. Smart training means smart recovery, so the goal should be feeling worked, but never drained. Weight lifting can do that well.
Grinding ourselves down in the gym is a moral desire, but athletic development is about simple progressive stress and recovery.
Also, that’s a reduction from what I use to do. In another life I was a part time fitness instructor and between classes, training for runs, and my own weight workouts,
I was exercising more than 10 hours a week.
You’re right. When people use to ask me how to lose weight, I would tell them it’s mostly diet. But I know myself. I’ve never been consistent about my diet but I like to exercise. I never cared about slim. Three days a week gives me the other four days to relax.