Step 5: Place your finger on filter #2, and swipe
BOOM. If you did it right, filter #3 has arrived. Now you've got a filter from all three classes. Nice work!
Bonus: FIVE filters (on videos only)
On videos, you'll have a fourth class of filters available: the ones that manipulate the speed and direction of the video. Repeating the same process, you should be able to apply: one color, one middle, one perimeter, slo-mo/fast-forward, AND reverse. Why? Mostly just because you can.
No matter how many filters you've applied, you can still write text (multiple lines of it, if you want!), draw, and use the free-form emoji. Go nuts.