3 letter format

Hy guys i have asked for help but it didnt work , please replay only if the correct soluction
i would to format cells a1:a10 so that if i enter for instance six letter


aaabbb then i want it to appear aaa-bbb

aaabbbccc , i want it to appear aaa-bbb-ccc



i have tried this formatting : ###-###-###-###
and didnt work


any help please

In B1 you can enter this formula:

=IF(A1="","",IF(LEN(A1)=6,LEFT(A1,3)&"-"&RIGHT(A1,3),IF(LEN(A1)=9,LEFT(A1,3)&"-"&MID(A1,4,3)&"-"&RIGHT(A1,3),A1)))

then copy down to B10.

Hope this helps.

Pete