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I have 3 Windows computers connected to the switch. The switch is not that great, it is a very cheap and basic 5 ports switch.

The Windows IPs are the following: 192.168.0.1 - 192.168.0.2 - 192.168.0.4

Debian Jessie is runnnig on the Windows 192.168.0.2 Virtual Box.

I want Debian to have this IP: 192.168.0.3

The switch has no Iternet connection.

I really need some help with this.

What mode is the Ethernet Adapter on the Debian guest? If it is NAT it will not get that address.

You might try Bridged Mode instead and then assigning the IP as you have for the other Windows hosts.