If you use a command, you could write it like this:
select htrans.field, htrans.immtype
from htrans inner join immun on
{fn left(htrans.servcode,3)} = {fn left(immun.immtype,3)}
Or you might be able to do this without the {fn }, and just use:
select htrans.field, htrans.field
from htrans inner join immun on
left(htrans.servcode,3) = left(immun.immtype,3)
...depending upon your datasource. The punctuation/syntax is datasource dependent as well.
You could alternatively manage this by using a subreport where you create a formula using the left() function and then link it to the corresponding formula in the main report in the subreport linking screen.
-LB