Step 3: Hold a finger on the screen
Use your other hand, and maintain contact with the screen. Then...
Step 4: Swipe to your second filter
There are currently three "classes" of filter: middle, perimeter, and whole-screen. You'll notice that if your first filter is from the former group, you'll only be able to access the latter groups for filter #2. (Otherwise, they'd fill the same space, and look like butt.)
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.