If this wikipedia page is to be believed, what you call mp3 and is actually a MPEG-1 audio layer 3 encoded audio file only supports three sample rates :
As @PkP has mentioned, besides MPEG-1 audio layer 3, there are more recent versions of the encoding, which allow other sample rates :
MPEG2 later (1995) added 24 kHz, 22050 Hz and 16 kHz. There seems to
also exist something called MPEG2.5 that also adds 8 kHz, 11025 Hz and
12 kHz, bringing the total list to nine possible sample rates.
Actually, the LAME encoder supports the following sample rates as argument of the
resample option (expressed in kHz) :
As the sample rate is written in the header of the file, I cannot see how you could have different sample rates in a single mp3 file.
As @PkP has mentioned, the sample rate is not written in the file header but for each mp3 block. Therefore is should be possible to have different sample rates in a mp3 bitstream.