static routes when multi-homed

I have a machine that has 3 NICs on it.  It is not a router and the plan
is to use static routes.  It running Solaris 8 (on Intel if that
matters).

What I'd like to do is have one NIC for inbound, one NIC for outbound,
and one NIC for a static route for private use.

I must be missing something obvious because I don't seem to be able to
find the right incantation of route commands to get this to work.

I'm not having any trouble with the inbound and outbound NIC setup.
I've got the application servers bound to the right address and it's
easy to setup a default address for the outbound NIC.

I want to be able to use the third NIC only to/from a known IP address,
i.e., I want to make sure I can get to the machine even when the other
two interfaces are "resource constrained."  But every time I try to add
a route to the IP address through the third NIC the machine won't
communicate with the desired IP address at all.

I've tried

    route add IP_destination IP_interface

Both with and without a metric, and with and without a /24 on the
IP_destination.

It can't be that this is not possible with static routes, is it?

Thanks!

Jim