FLAC is lossless, so there's nothing lost from going FLAC->Wave* (er, FLAC->PCM - Wave is a container, not a compression format) but it depends on how that FLAC file was created. Doing MP3->FLAC->PCM is no different than just doing MP3->PCM.
*unless you're also changing things like # of channels or the audio frequency, of course - but if the channels and frequency are left alone, then no loss happens
When it comes to MP3->PCM, the loss isn't incurred when you convert to PCM. It's the fact that the data was lost when the MP3 itself was created from whatever source it was encoded from. You can't get quality back after a lossy conversion.
So, blind response, FLAC->PCM would be the 'best' option, if you are certain there was no lossy conversion in there beforehand (like, for instance, you ripped it off the CD yourself or it was distributed by the band or label in FLAC form).
In diagram form:
PCM on CD (lossless, because of the same reasons Uncompressed anything is technically 'lossless') -> FLAC (lossless) -> PCM (again, lossless, and should be mathematically the same as the first PCM file, weird error correction/tagging issues notwithstanding)
PCM on CD -> MP3 (lossy) -> PCM (lossless, but because it passed through MP3 first, it will not be mathematically the same as it was on the CD; whether you hear the loss of that data is the same as whether you could hear the loss in the MP3 file, though - you can't get better than what the MP3, or lowest compression method**, sounded like in this case)
**i.e. the PCM on a CD is probably (hopefully) not as high fidelity as what the technicians in the mastering studio/plant were working with, but this is a given. If you're working off a CD, then the FLAC->PCM is likewise limited by what the CD sounds like. Or if you chained a bunch of lossy generations together, sure it would sound like crap, but it would sound like the crap the very last encode in that sequence sounded like. Say, PCM on CD->MP3->WMA->AAC/MP4->Vorbis->AC3->Vorbis again->MP3 again->PCM, it could only sound as good as that final MP3 encode before going to PCM does.