So, you have made up your mind about tri, and started training. Here is a progression from the beginning to where it may end up going.
First, you will likely do one thing a day (like a run or some biking) with some rest days every third or fourth day perhaps.
Next, you will have some days when you do two sports, more of one, a bit of the other. You will rest some days still.
If you intend to go beyond this point, you will probably want to hire a coach (if you haven’t done so yet). Or use training plans available online or in books. It is because you start to spend real time training and you will be probably unsure about how to optimize and structure all that time invested. A coach or a good plan can help. My vote goes to the former, though.
Third, you will probably find yourself doing two things most days, and eventually the rest days will be gone.
Fourth, you will be acquainted with the notion of “long days”. These will usually be weekend days, when you will do a lot of one or two things. Like a really long ride, followed by a real run.
Fifth, if you had not done so before, this is when you will start to add massage, chiropractor, yoga, Pilates, core strengthening exercises into the mix. All that training will make such preventive maintenance necessary.
You are training hard at this point, doing real work every day, with great plans for big races and PBs. Most importantly, you will be getting that feeling of profound satisfaction, every time, all the time when training is done.