Here's one way, using an array, that is suitable for any number of scoring_var# columns you have.
data have;
input customer_id $12. scoring_var1 scoring_var2 scoring_var3;
datalines;
CUST0000001 0.98765 0.01234 0.99999
CUST0000002 0.87654 0.76543 0.54321
;
run;
data want;
set have;
array scoring_columns scoring_var:;
max = max(of scoring_columns[*]);
run;
For more details, or to see a few other methods, see this link.