OpenBSD route via different gateways which have same IP but on different interfaces

Solution 1:

Use the -ifp modifier to route. From the man page:

In a change or add command where the destination and gateway are not
sufficient to specify the route, the -ifp or -ifa modifiers may be 
used to determine the interface name or interface address.

So something like this works:

# for  arg in tun0 tun1 tun2; do ifconfig $arg  192.168.11.1/24; done
# route add 10/8 -iface 192.168.11.1 -ifp tun0
add net 10/8: gateway 192.168.11.1
# route add 172.16/12 -iface 192.168.11.1 -ifp tun1
add net 172.16/12: gateway 192.168.11.1
# route add 192.168.254/24 -iface 192.168.11.1 -ifp tun2
add net 192.168.254/24: gateway 192.168.11.1
# route show -inet
Routing tables

Internet:
Destination        Gateway            Flags   Refs      Use   Mtu  Prio 
Iface
10/8               192.168.11.1       GS         0        0     -     8 tun0
localhost          localhost          UHl        0       22 32768     1 lo0
172.16/12          192.168.11.1       S          0        0     -     8 tun1
192.168.11.1       192.168.11.1       UHhl       1        4     -     1 tun0
[...my real routes omitted...]
192.168.254/24     192.168.11.1       S          0        0     -     8 tun2

If your destination routes are overlapping then you can use pf and route labels to match, or routing domains.