The first command should run synchronously...
If you start the two last commands as background processes (add & at
the end of the command), they will run in parallel.
Do you want the script to wait for the two last processes to be complete
Something like this should work:
sqlplus user1/pw @/home/scripts/tst1.sql
sqlplus user1/pw @/home/scripts/tst2.sql &
sqlplus user1/pw @/home/scripts/tst3.sql &
# while (`ps -p"$pid2,$pid3" | wc -l` != "1");
sqlplus user1/pw @/home/scripts/tst4.sql