403(b) is subject to universal availability. However, one the exceptions to the universal availability requirements is to exclude any employee who is eligible to defer under another vehicle (i.e. 401(k) or 457(b)). So, by writing the 401(k) to exclude the CEO, CFO, and Director of Finance, you achieve two things:
Have them excluded from the ADP test in the 401(k) plan while having them as the only employees who may defer under the 403(b) plan.
I cannot detect an issue on the surface.
Good Luck!