To add to @SAS_Rob , I like to use the options LIST and MISSING in a tables statement in Proc Freq when more than 2 variables are involved as the output is easier to read and is pretty clear about where the "missing" are impacting.
Dummy code as an example:
Proc freq data=yourdata;
tables var1 * var2 * othervar / list missing;
run;
This would be recommended for Class or categorical variables.
... View more