Does (muscle confusion/body adapting to exercises) apply to Cardio?

It is my understanding that we are fooling the brain and not the muscles. In the old days we walked everywhere and carried our bundles. As a society in general we are used to jumping in our cars. My grandparents lived to a ripe old with minimal disease due to the fact they walked a lot. People were not focused on exercise back then, just living and raising families. It was a different time. With all our modern conviences we do not get the exercise we should.

The cardio equivalent is called intervals.

First of all, changing your routine only once every three months is not good. It is fine for a beginner (people who have been lifting for less than a year). However, for the more experienced lifter, in order to improve muscle gains, routines can be switched up more than once a week (for more extreme, experienced lifters).

As far as the cardio, your body will not lose less calories just because it has adapted to running for x miles a day. A certain amount of calories will be burned everytime you run x amount of miles. The key thing here is how hard you run and how long you will be running hard. The faster you run, the more calories you will burn.

In this case, you could even run 1 mile less if you were running very hard at a consistent pace and burn more calories than running at a slower pace for 3 miles. Intensity is everything in burning calories. The more intense the workout and the longer the intensity lasts, the more calories will be burned. Hope this is clear enough for you.

your body never adapts to the amount of calories burned. It will always burn calories at a certain rate depending on what kind of calories you have consumed and how intense/not intense your work bouts are

Yes; I think the interval training counsel is good, especially re cardio.
For cardio, you need to put the heart under pressure: it is basically a muscle, like any other. But it is put under pressure as it is taken from high output to low output back to high output, etc
So, you need to raise its work levels for a sustained period – a minute or two, then rest and let the heart rate slow down… then put it under pressure for another minute… then rest again… and so forth.
As another mentioned, you will also likely find that this greater intensity of work over 20+ mins will burn off more calories than an hour of flat walking, because mathematically, the greater power variations for this kind of workout require an exponentially much greater expenditure of calories

As for whether you get fatter again if you simply continue to work at a certain [adapted] level: I think it would be mainly that you would not progress in calorie/weight loss beyond that point. But having said that, perhaps you could consider the following as well:
Because you would now weigh less and because your body’s structure had now adapted to more efficiently performing that exercise, you may find that it would not provide quite the same benefit as previously. So it would likely be that to maintain a weight or fitness level, you might have to alter your activity to one the body is not so adapted to, or increase your intensity [which most people would do naturally], or the length of time over which you exercise.