I don't know much about networking so I hope this question is clear.

I have two laptops that are connected to the internet and another computer, my DAW, which is off the internet (except when updating Sonar).

I've set up a Homegroup to network the two laptops to each other.
What I'd like to do is connect the DAW (via the Homegroup network) to the laptops, but without exposing the DAW to the internet.
Is there a way to do this? 

Unplug your cable from the cable box. 
 
Maybe too cryptic. You are connected to the internet via some type of interface, if it is a TV cable service it is a cable modem, if a telephone line service it is a DSL modem. Sometimes this interface is integrated with the router that connects your computers to it and to each other, either by a wired or radio connection. In any case there is a physical connection between that interface and your telephone line or cable. If you break the connection between the line connecting that interface to the outside internet you are completely disconnected from the internet. Your router will still connect your home computers to each other. There are usually more elegant ways to achieve the same effect in software, but without knowing specifics of your setup they are not clear. Breaking the connection is usually pretty easy and it is foolproof and unhackable. If you plan to do it frequently, there are switches that can be interposed between the internet connection and the router. 

disconnecting the router from the outside world is a form of "air gap", and it is easy, and it is effective, and if you have  no need for internet access, or at least no need for internet access while the DAW is connected to the laptop then it will work.
 
There is a far more elegant solution however... you can simply block (deny) all traffic to and from that computer in your router. The details will depend on the make and model of the router - you could call support at your ISP or the manufacturer for an explanation.

Once blocked the DAW will see the local computers but it won't see the internet.

 
Thanks slartabartfast. I didn't know about a switch that could be placed between the modem and the router. I'll look into that.

And thanks wst3 for your suggestion. I'm pretty sure I can set up the router like you described. I'll give it a try.

But I'll probably find that it's easier to just get out of my chair to plug in/out the cable from the back of the DAW like I've been doing.  Leave it unplugged most of the time. Plug it in when I need to transfer a file from another computer on my home network or to download something from the internet.
 
I appreciate the responses.

I want to add a "software" solutions:
1) if you disable/enable the interface the effect is close to unplugging the cable.
2) if you configure the IP for the interface statically, you can remove the Gateway setting. If no other computers/software has "share Internet" features, effectively your computer will "forget" how to connect to the Internet. That is quite some click initially, but once you have done that once you will be able to restore/disable Internet within 20-30 seconds. It is important to assign "right" IP address, otherwise your router can give the same address to another device coursing fancy problems, so that "right" address should be either from the range your router never use or you configure the router to always assigned the same address to this particular computer.
 
Software solutions are more tricky, but hardware solutions (cable, firewall inside router, etc.) have at least one side effect: as soon as Windows "think" it is connected to the Internet, it tries to update/"call home"/sync storage/etc. So it starts some activity. It will fail when blocked on the other end of the wire, but it will try again, and again, and again... I mean it will restart corresponding "service tasks", possibly interfering with the DAW.
F.e. you shortly want transfer some file, connect the cable and Windows immediately start all possible "updaters". Even after you unplug the cable it can take a while till all that programs stop eating resources.

Thanks azslow3.  I get the gist of what you're saying and I might be able to figure out the router settings you described, but I'm not feeling too confident about it. That was a good point about Windows calling home and starting updates during those times it's connected briefly to the internet. Always wondered about that.