The gateway 192.168.2.100 is on a different subnet from the client computers. It is on the 192.168.2.0 network. The client computers are on 192.168.3.0, 192.168.5.0 & 192.168.6.0 networks. You will need to have a gateway with addresses on each of the these subnets to route between them.

So, you should have a gateway device with addresses like

192.168.3.254/24
192.168.5.254/24
192.168.8.254/24

Each client NIC will have the appropriate gateway address from the list above.

The default route for unknown traffic will be the interface with the internet or outside connection.

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As LabNuke mentioned, your PC’s NICs are on different Subnets / LANS than the listed gateways.

First PC:
First network card
IP 192.168.3.106
Sub 255.255.255.0
Gateway 192.168.3.1

Second network card
IP 192.168.5.106
Sub 255.255.255.0
Gateway 192.168.5.1

Second PC
First network card
IP 192.168.6.106
Sub 255.255.255.0
Gateway 192.168.6.1

Router – Good that its on a seperate “management Network”
IP 192.168.2.100
Sub 255.255.255.0

The Netgear you mentioned you’re using may NOT support multiple subnets/VLANS (most home / end-user routers don’t)

So the routes you set up are 1) uneccessary and 2) incorrect. If your router is connected to your ISP modem, then your gateway (for the router) would be the modems IP address:

ISP Modem: (example) 192.168.1.1
WAN IP on router: 192.168.1.100

That would make the routers “gateway” 192.168.1.1

But, to address the PC’s, IF your router CAN do more that 1 subnet, then you COULD create the VLANs/subnets on your router, assign a specific port to each VLAN/subnet, and cable the appropriate PC NIC to it’s given subnets’ Port (again, a lot of home router don’t support that…)