The fact that you ask the question, you are already half way there.

Here are a few steps I advise my customers to take:

1. Find the three core values all three founders share (easier said then done).
2. When recruiting, look for a sales person who shares those values.
3. Make sure the compensation plan rewards those values.
4. Train one of your founders to become a sales manager
5. Agree what target is non acceptable, acceptable, & realistic (again, easier said then done).
6. 100% commission for reaching the realistic target, 110% for over achieving, 70% for reaching the acceptable target & 0% for anything below.
7. Last but not least, have a 2 year career path in place.

Hope this helps.

PS: hiring and firing sales people will not only cost you time & money, it will also damage your brand.

Steve