Routing traffic on multiple nics

Now you need to know the IP address where the y.y.y.y is hiding behind and lets call that ip yyyrtr
same for all other addresses. After that one can use:

route add y.y.y.y gw yyyrtr
route add b.b.b.b gw bbbrtr
route add z.z.z.z gw zzzrtr
route add q.q.q.q gw qqqrtr

A route is not needed if the interface is on that network...  (bringing up the interface also establishes a route to all addresses there).

So if the address of eth2 happens to be q.q.q.1 then no route is needed.  (assuming q.q.q.q is within the network descibed there).
See also:
https://en.wikipedia.org/wiki/IP_routing
https://www.youtube.com/watch?v=8qtKpZGoNdI

Aside: If you're running LXD on your machine, the SNAP update process causes all LXD related bridge adapters to be deleted.

So if you're using LXD, you must arrange for your route commands to regenerate every few seconds.

Can we do that using iptables?

No you use the routing command:  route

route add {destinationhost} gw {nexthop}
or
route add -net {destinationnet} gw {nexthop}

{destinationhost} is one IP addressi
{destinationnet} is a network specification like:  
{nexthop} the next system that better knowns were to go.

If those work they need to e added to the system configuration...
In the directory: /etc/sysconfig/network-scripts/
You create a  file / device.  for eth1   : route-eth1
Containing:

y.y.y.y  via routeryyy dev eth1
a.a.a.a via routeraaa dev eth1

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All data needs to be in ipaddress format, not a hostname
etc.