With a plug-in sampler that support those formats. I do not know of a sampler that supports MP3 but it is easy to convert Mp3 to aif. There are many sampler plug-ins out there (e.g. Sampletank 2, VSamp, etc.)
I use VSamp - it's cheap, stable, and easy on the processor. It can use AIFF files natively - to use MP3s, convert them to AIFF files in iTunes. If for some reason you must create a soundfont file, your options are limited. The only one I know of for OSX is PolyPhontics:
Yes, it reads .wav, .aiff, and sdII formats. You can also load soundfonts (but not create them). It is very easy to map samples accross the keyboard and you can layer samples for velocity switching, etc. Not bad for $39. You can read more about it at: