No need to play with the routes if you're under Windows (as it seems to be). Windows handles natively multiple IP configurations for the same network interface.
To do this, open the Network center (not sure about the English name for it), then click on the second link on the left to go into the network interfaces configuration panel (same). You might have more interfaces than me, but it's not relevant.
Right-click on the interface where all your networks are connected, then choose "Properties". Here, search for the line "Internet Protocol version 4 (TCP/IPv4)", or something like that. Double-click on that line. In the new window, click on "Advanced".
In the new window, you will be able to input :
Your three personal IPs. Either you're using static IPs or DHCP. If you're using DHCP, it should be displayed as "DHCP enabled". In this case, you cannot add IPs and it's fine. Otherwise, simply add your personal IPs.
The three gateways, one for each network. Difference is, you also have to precise the metric for each one. The metric means the priority of the gateway. The highest will be the default one. So just set the Internet GW to 1000 and the two others to 100. This way, you will always use the Internet GW for your regular browsing, but you'll also be able to access the two other networks normally.
Finally, just click OK, OK again, and OK one last time. You should now be able to access every network at the same time