New to Magic Kingdom - how to get more Fastpasses?

Visiting MK with our 3 kids for first time in April. I went online and got tickets and 3 fastpasses for the day. Question I have is this: when can I get more fastpasses? After I use just 1 or all 3? And how do I get them?

You’ll be able to get more FastPasses after you use your existing 3. Once you use your last FastPass, you’ll be able to book new FastPasses one at a time.

After you check in for your third, check for a new fast pass using mde. Rinse and repeat.