Sonicwall routing between multiple subnets on multiple interfaces
Devices in 192.168.3.0 need to know that they can reach devices in 192.168.2.0 via 192.168.3.254. The way they'll know that is to create a route on "Another Router" for 192.168.2.0 via 192.168.3.254.
EDIT
When you ping or connect to a device in 192.168.3.0 the return traffic has to have a path back to the device in 192.168.2.0 that originated the traffic. That path needs to go through 192.168.3.254. By default all non-local traffic in 192.168.3.0 will be forwarded to "Another Router" since it's the default gateway for hosts in the 192.168.3.0 network. Once you have the route configured in "Another Router" you need to create a firewall rule on the Sonicwall that blocks traffic originating in 192.168.3.0 from accessing 192.168.2.0. When traffic originates from 192.168.2.0 the return traffic will be allowed through the firewall since it originated in 192.168.2.0.
As recommended by David Schwartz, the way I solved this problem was to create a NAT entry in the SonicWall that translated the "Source Address" from the 192.168.2.0/24 network to the SonicWall's interface address on the 192.168.1.0/24 network. Then I allowed traffic to go from all LAN subnets on the sonicwall to the X3 subnet. Works like a charm.
(I meant to post this answer a while ago; never got around to it).
it might be late for RAIN, but as I was among the people who still faced this issue, I thought it might help others also to learn another more direct approach on this setup:
http://www.blizzardcomputers.com/multihome-lan-sonicwall/
Example for 192.168.10.1/24
Under Network Menu Click Address Objects.
Name: My Secondary Subnet (or any friendly name)
Zone Assignment: LAN
Type: Network
Network: 192.168.10.1
Netmask: 255.255.255.0 (This is for a /24 change as needed)
Click OK
Under Network Menu Click ARP
Static Arp entries > ADD
IP Address: 192.168.10.1 (use the IP you want to be your default gateway for this subnet)
Interface: X0 (Choose your current LAN port)
[x] Publish Entry
[ ] Bind Mac Address (Do not select)
Click OK
Under Network Menu Click Routing
Under Route Policies choose Add
Source: Any
Destination: My Secondary Subnet (Or whatever you named your new Address Object)
Service: Any
Gateway: 0.0.0.0
Interface: X0 (or the port you chose in ARP setup if your LAN port was not X0)
Metric: 20
Click OK