Routing OLTP users to use particular instances in RAC environment

We have 3 node RAC setup with DSS & OLTP type of users. We have restricted the
DSS users to use only one instance(SFL3) through ONLOGON TRIGGER.
OLTP users
are coming through FORMS6i menu to RAC1, RAC2 & RAC3 using LOAD BALANCING.

Now our requirement is restricting the OLTP FORMS users from accessing RAC3 server.

That is they should only use RAC1 & RAC2 servers through LOAD BALANCING.

Create another tnsnames.ora alias that only points to the two you want them to use.