
most turntables require a preamp to work, so unless your turntable has a switch for line / phono (most are phono) you will have to have it connected to the mixer, which has a preamp.
so turntable into one phono channel on mixer.
mpc into same channel of that mixer, but on line input.
zoom drum machine into the other channel of the mixer.
mixer rec output into mpc line in.
the whole mixer into the sp-303 input.
sp-303 output to your speakers.
so when you want to sample you will flip the mixer channel 1 to phono, plug the headphones into your mpc (you wont be able to hear it out the speakers), and you use the mixer to send a signal to your mpc.
when you've got samples in there ready to play, you can flip channel 1 on the mixer back to line and now your mpc will come out the entire mixer.
you can use the sp-303 as a master effects unit too now.
that is only one of a 1000 different setups you could use, and it is by no means the 'right' set up.
i think ill-green's idea is also just as good, just for different reasons.
if your turntable has line output i would plug it directly into the mpc.
if you really want to have everything connected in the most optimal way, you need a bigger / better mixer.