networking - Linux: multiple network connections - 3G/4G / Wifi / LAN / etc; how can i set a preferred network connection to use?

I've been looking at how I can setup a laptop that has multiple network interfaces, but a problem exists if all the connections are active, i.e. 3G, WiFi and LAN are all connected, I would like it to default to LAN.

I would like to set "weights" or "priority" to each connection, so that if the LAN is unplugged, it'll default to WiFi - if its in range and working, otherwise, it'll switch and use the 3G dongle;

I've been looking around and I can see that the "metric" counter for route isn't being used for recent kernels. I thought that would be able to set the preferred gateway / connections - but according to the man page:

man route:

OUTPUT

Metric The 'distance' to the target (usually counted in hops). It is not used by recent kernels, but may be needed by routing daemons.

So I'm confused, are there any scripts / apps / anything that can detect active network connections, and by way of configuration, send my default gateway network traffic through that interface if its active / alive?