How do you want your final output--as a dataset, or a printed value?
Here's one way to do it--counting the total number of rows from those three datasets. This code will save the final sum to a macro variable, and print it.
data table1;
input columnA;
datalines;
12
10
5
6
;
run;
data table2;
input columnB;
datalines;
8
13
34
;
run;
data table3;
input columnC;
datalines;
20
2
24
1
17
;
run;
/* Combine table1, table2 and table3 */
data have;
set table1 table2 table3;
run;
/* Put the total Final Sum in a macro variable */
proc sql;
select count(*) into :final_sum
from have;
quit;
%put The Final Sum is: &final_sum.;