I have an AC500 with one B300S and two B300, can I add a B300K and will it work efficiently?
Yes, but you will need the additional P090D To P150D cable if you connect it directly to the AC500 or B300S like you do with the B300. Depending on the firmware of the B300 you are either limited to 4 or 6 batteries, either way you will be fine with 4. When you go above 4 you need the B300 batteries to be within the first 4 batteries in the ordering.
Edit - That change from 4 - 6 batteries when using B300 batteries might be Apex 300 specific so take that with a pinch of salt. However with 4 batteries you will be fine.