tell me if I got this straight.
you are using three VM installs of Debian.
1. is a router to be used by the other two to get into the world outside, and to each other?
2. the router is used for the main gateway to the outside internet?

or

they all get outside internet by themselves and the router is used for the other two to talk to each other?

https://www.virtualbox.org/manual/ch06.html

I have the following vbox and images:

- VirtualBox Graphical User Interface
Version 5.1.8 r111374 (Qt5.5.1)

- debian-8.6.0-amd64-netinst

well I could not get Debian installed on my VM due to them having mirror issues gaining access to complete the installs SO I am using Slackware instead. I am just in the beginning stage of post install on all three of the VM installs of Slackware. it should not matter the network would still be needed to be set up the same.

look at the photo I drew of the schematic I think you're trying to say you want. but it is defeating the router.

please explain in detail what are are trying to accomplish here.

from your earlier post, and taking from everything you said on what you're trying to do, using only two. You should have had it done by now. so I am sketchy on what it is you are trying to do here.

that gives you three.. and you're having trouble getting two VM talking to each other? mine just does using a bridge connection.

Attached Thumbnails

 

take a look at this too
it shows how to set up a network environment within VM routers and clients and everything as such
http://www.brianlinkletter.com/how-t...ate-a-network/

NAT routing


VM Ware

Routing between Two Host-Only Networks