Have you purchased support? If so, call them.
Otherwise, these forums are free and voluntary. It is not our job to search the forums for your hardware specs. We have jobs and lives outside of this forum. Very few people here, (incuding Sky-Knight) work for Untangle, so if you don't want their help, you're going about it the correct way. If you do, show some courtesy.
You did give us one clue. They are all "older dells" ... what a shocker if, as has already been pointed out in this forum, there was an incompatible piece of hardware that was the same in all three? Or that, heaven forbid, there was an actual problem with h/w... I had two Dells in a row have a bad hard drive, and then a bad NIC. Both were "good machines" in their previous life.