Select LastName from MyTable

Where LastName like '[A-C]%'

Select LastName from MyTable

Where LastName like '[D-F]%'

or

Select LastName from MyTable

Where LastName like '[ABC]%'

Select LastName from MyTable

Where LastName like '[DEF]%'

...

It would be good to have an index on LastName. Check execution plan and logical reads.

Hope this helps.