What are the next three in this sequence 21f11f01f9f?

The sequence appears to consist of pairs of numbers followed by the letter "f." The numbers decrease by 2 each time: 21, 11, 01 (which can be interpreted as 1). Continuing this pattern, the next three numbers would be -9, -19, and -29. Therefore, the next three terms in the sequence would be -9f, -19f, and -29f.