Setting the triple state property to NO as cap1 suggested will work.
cyberkiwi's suggestion can also be modified to accomodate NULLs (which you get in a triple state check box that has not been used yet - showing up as grey instead of checked or clear):
If nz(CHK1,0) + nz(CHK2,0) + nz(CHK3,0) = 0 Then
The NZ function checks for NULL values, and substitutes some value (in this case 0) for nulls when they are found.