How can I add a route for good on a machine so that when it boots up it has that route in its routing table?

Add the 'route' line to either your network startup script, or rc.local file...

Of course how elegantly you can do this depends on your distro.

I use SUSE. would I add this to boot.local

would this work in boot.local

route add -net 192.168.3.0 netmask 255.255.255.128 dev eth0

or is there another format?

Linux usually adds directly connected networks to the routing table automatically. If eth0 has an IP address on the 192.168.3.0 network then it should show up in the routing table.

If the network is not directly connected, then you need to specify the router that is connected to it, not just the interface that is connected to that router:
Oh and FYI, if these are the numbers for your actual network, they may not work. 192.168.x.y is a class C network and it needs to be subnetted into at least 4 networks, since you can't use the first and last. 255.255.255.128 creates two subnets, minus the first and last leaves none.

It works. Why would it work if I dont have enough networks?