If you don't have experience managing salespeople, don't -- under any circumstances -- hire someone into a non-existent role.

A better approach is for one of the three founders to take on the sales role, develop it, document it and benchmark it.

If you're concerned that the founders can't sell, don't be. Founders can always sell -- because they are founders. (If your founders can't sell your product, you have a product problem, not a sales problem.)

When you hire your first employee, pay them a salary (not commission) and have them work side-by-side with your founder-salesperson until they are able to equal or better your founder's numbers. At that point they are trained.

They should be held accountable to your founder's benchmarks until they better them (at which point the bar shifts). Salary should be determined by market value (not commission) and performance should not be optional.

You should monitor the number of meaningful interactions a day (in addition to sales, of course). You'll quickly get a feel for the optimal number (probably around 45 a day) and once you know, this volume of activity should be compulsory. Activity isn't everything, but without activity, you have nothing!