how to monitor mobile data usage but exclude wi-fi I'm aware there are a large number of data monitoring apps for android but as far as I can see, they all include wi-fi usage in the total.

Since my wi-fi usage is unmetered, I only want to monitor 3g mobile data usage. is there an app that can do this?
Try My Data Manager.  It monitors network and Wi-Fi data.  If you have at least Android 4.0 it should also allow you to view the data use directly without having to install any additional apps.