Don't be lazy. Configure it completely.

auto eth0                      #add this line
allow-hotplug eth0
iface eth0 inet static
        address 192.168.3.10
        netmask 255.255.255.0
        gateway 192.168.3.1
        network 192.168.3.0
        broadcast 192.168.3.255

Warning! For this to work properly dhcpcd needs to be disabled. See How do I set up networking/WiFi/Static IP

Don't be lazy. Configure it completely.

auto eth0                      #add this line
allow-hotplug eth0
iface eth0 inet static
        address 192.168.3.10
        netmask 255.255.255.0
        gateway 192.168.3.1
        network 192.168.3.0
        broadcast 192.168.3.255