It sounds like the USB drive doesn't have the drivers to boot the other macs. :-(
The fastest path forward is to simply erase install a basic Snow Leopard OS onto the failed macs. While this is happening, do download the 10.6.8 Combo update to your USB drive just in case it's needed.
Don't bother running the updates unless the Lion installer forces you to get to a higher version than your 10.6 installer delivered.
Once you boot into Snow Leopard - you can try again to mount the USB and execute the Lion upgrade package. I don't know if it runs well from the USB or needs to be copied to the internal boot drive.
There's a little uncertainty in my brain - so I don't want to write too much without making sure this makes sense to you.
As an alternative - you could try instead to transfer the recovery partition, but this may not be universal (include the drivers the older macs) either.
There is a step-by-step recipe here for copying any bootable volume to one file on a USB drive.
If you are curious or feel it's worth a shot, image the recovery partition from your Lion mac.
You should be able to boot from DVD and use disk utility to make an equivalent partition on the "non bootable" mac and drop the recovery data to get a minimal bootable system and avoid a full Snow Leopard install.
Unless you are familiar with Disk Utility and the steps to capture, the reinstall option might be more likely to succeed on first attempt. I certainly don't know if this partition is customized by Lion and not universal so I've made it an aside for the curious.
I called 1-800-MY-APPLE and bought Lion 10.7 for only $19. I told the automated machine that I wanted to buy OS X 10.7 Lion. I was transferred to a sales representative, and he looked it up and said it was $19.99 plus taxes. I was surprised because I was expecting a $29.99 price tag. So I made him double check to make sure it was indeed 10.7 Lion and not 10.8 Mountain Lion. He said that it was for sure and that I would be emailed a PDF with a redemption code to get it.
I went through with the purchase last night (November 6, 2012) and got the email today. I was able to redeem the code in the App Store and download it from there!
Best Answer
I always do option 3. You should be able to pretty much follow these directions, even though they reference Lion: http://forums.macrumors.com/showthread.php?t=1189888
Long story short: When you buy an app from the app store it is stored on your drive in a format that can be burnt.
Of course, the USB drive option is also doable and it sounds like that would be easier for you.