What impedance is your sub?

The only way to run a pair of speakers and a sub off of a 2 channel amp would be to run it in "tri-mode" (if the amp is capable of doing so) with the use of passive crossovers.

This is where you send a full signal to the amp, run your 2 speakers in stereo on the R & L channels with a HP filter and then bridge those 2 channels with a sub with a LP filter.

You'll be much better off with 2 amps though.