How to set gateway of last resort in this WNDR3700.

You probably need to add two static routes: a 0/0 route on R0 pointing to L0 and another 0/0 route on L0 pointing to R1.  But traffic from devices on the 192.168.3.0 subnet is going to take an inefficient path through R0 to get to either the Internet or subnet 192.168.1.0, so I would not recommend doing this.

What you really want is for R0 to advertise 192.168.3.2 as the default gateway in DHCP.  This way, devices on that subnet will send traffic directly to L0.  L0 would still need a static 0/0 route pointing to R1.  Unfortunately, the WNDR3700 can only advertise itself as the default gateway, so this is not an option.

Here are some options for you to consider.

Option 1: Swap R0 and L0

Set up R0 as an Access Point and wire it into R1 via a LAN port.  Disable the DHCP server.  This eliminates the 192.168.3.0 subnet.  Everything would see R1 as the default gateway.  I suspect that this not an option for you because R0 is probably positioned where it is to provide Wi-Fi coverage, or you are using L0 as a firewall and it has better capabilities than R0, or you can't/won't run Ethernet to R1.

Option 2: Replace L0 with a switch

Install a switch where L0 is located and wire it R1.  Same considerations as option 1 except that this option allows you to keep the physical location of R0 unchanged.

Option 3: Use L0 as a DHCP server

Disable the DHCP server on R0 and enable it on L0 for the 192.168.3.0 subnet.  Devices on that subnet will then send traffic to L0.  L0 still needs a default route pointing to R1, but it should get that automatically via DHCP.  If you are not a techie, setting up a DHCP server on Linux may be hard.  dnsmasq is the standard option.

Option 4: Set up bridging on L0

The idea is to bridge the two interfaces on L0.  This eliminates the 192.168.3.0 subnet, so disable DHCP on R0.  This may be challenging to set up.

Option 5: Replace L0 with a range extender

Same as option 4 really but with less fuss.

Option 6: Install DD-WRT on R0

DD-WRT can easily advertise L0 as the default gateway in DHCP.  This is probably just as difficult as setting up a DHCP server.

Option 7: Use static IP addressing on subnet 192.168.3.0

With static IPs you can easily set the default gateway on all devices to point to L0.  Obviously, this is very cumbersome so I only mention it for completeness.