Can this flash gun be used on a Nikon d3000?

Metz 44 AF-1 Digital Flashgun for Nikon

I would say given that it is a DIGITAL flash, that the trigger voltage will be safe for a modern DSLR. In fact, on looking it up - it has a trigger voltage of 4.7 volts - which is well below the limit for Nikon DSLR's.

Yes it can be used - apparently it has TTL mode (auto flash metering) as long as you buy the Nikon version. Alternatively, you can use it in manual mode (camera in manual too).

Metz makes good stuff. You should have no issues using this flash for your D3000. However, it can't be used as a wireless remote flash as your D3000 does not have the capability to remote control a flash.