Try this on UnixA. Update your local routing table as such:
This should send anything destined to the remote 192.168.3.0 network to routerA (rather than sending it from UnixA to LinuxA then to routerA). I'm assuming that 192.168.3.0 has a standard 255.255.255.0 netmask. If not just adjust the above code.
route add -net 192.168.3.0 netmask 255.255.255.0 gw 192.168.100.101 metric 1
If LinuxA is the default gateway for your entire LAN, and everything destined for LocationX, not just traffic from UnixA, needs to go through your VPN box, then just add that route to LinuxA and set all your LAN box to use LinuxA as their default gateway. However, if you need to maintain static routing tables on a lot of boxes, you should look into setting up something like routed.