The first thing to do is to record all the fixed IP addresses and the MAC addresses of each device. You can configure reservations for these devices so they can have the same fixed IP address. You don’t necessarily have to change the subnet mask, you just need to have a layer 3 (router) device to route traffic between the /24 subnets. By having all an address on all 3 subnets on the same NIC on the server, you can easily route between subnets. Or, implement another device that can have 3 addresses, one on each subnet and can route between the networks.
You will also need to adjust your DHCP server settings so that it can not only make reservations if needed, but to change your address range that it’s dishing out.
You can keep the subnet the same 255.255.255.0, and just increase your address range to lease 192.168.101.0 – through – 192.168.103.255 or 192.168.101.0 – through – 192.168.102.255
Great idea from the other guy to document your stations MAC and IP addresses, that could be very useful in these tasks, troubleshooting, and future expansion.
Hope that helps