The formatter is a placeholder.

If you imagine a coaster (as in the little bit of card where you put your cup when you're drinking something...) That's comparable to a formatter, in the sense that it shows the place you should put something.

There are only two formatters here:
"%-15s%03d%n"
The one that you think is the third: %n actually represents a linebreak (like hitting Return on your keyboard while typing a sentence).

%s = String        // a string goes here
%d = int (digit)   // an int goes here
%f = float         // a float goes here

Let's look at the whole statement:

("%-15s%03d%n", s1, x)

The two formatters are:
%-15s which is where the variable s1 (a String) is placed.
%03d which is where the variable x (an int) is placed.

The other parts of those formatters mean different things...

The -15 part means that:
-: when s1 is put into this spot, keep it to the left side.
15: make sure it takes up 15 spaces.
EXAMPLE if s1 = hi then it will produce:

"hi             "  // Note that the string is 15 characters long

The 03 part means that when x is put into this spot:
3: it must take up three spaces.
0: any spaces it doesn't use are to be filled with 0's.
N.B. The default padder is whitespace " " if 0 is not used.
EXAMPLE
if x = 70 then it will produce:

"070"  // The result is 3 characters long, with a '0' filling the unused space

Time to watch some tutorial videos I guess! Good luck.