The BML diet referenced in the Gliderpedia (above) is one that doesn't require anything that you can't get at your grocery store and a pet store. It does need to be followed exactly as written but will be easy and cost effective until you get your shipment in. After being on their current "diet" for so long they may take a day or two to get used to eating something different. But, there is no reason to wait to get them on the path to better health. There also shouldn't be an issue of switching them over to the HPW when you get it. Maybe a 3 day period that you'll notice.
The poor little guys are lucky you've found them! I hope everything goes well at their well check with the vet! 3 years is a long time to have been kept that way.
Congrats and welcome to gliderhood!