I'm sorry for your loss.
I recommend cutting a 2 or 4 rectangles, 3" X 6" - 5" X 10" in size of fleece. Place one rectangle of fleece in your pairs sleeping pouch and one in your loner's pouch. After 1-2 days take out the rectangles, lay one on top of the other and cut into half. You should have a square size from each pouch layered. You can then place the squares from each in pairs sleeping pouch and one in the loner's sleeping pouch directly, or place them into a ziploc sandwich bag together, making sure no air inside, and do the 2nd set of rectangles if you made them the same as the first set.
Having the mingled scents from each glider's cage in the fleece squares will get them accustomed to their "mingled" scent, hopefully creating a "colonized" scent of the three of them.
After one week of the "mingled" scenting procedure, place the pair into one bonding
bag (with a mesh window) and your loner in a separate bonding
bag (with a mesh window). Wear them together, screen to screen as often and for as long as possible during the day. Try, if you can to do this at different times of the day example: early morning after just going to sleep, then about mid day, and then early evening before meal time. You can do this on different days if your schedule permits, but no worries if you only have a window of the same time daily. Do this for 1-2 weeks, watching and listening for reactions. In the meantime, keep the mingled scented fleece in their sleeping pouches
If within the one or two week period, they seem to be relaxing or at best, no hostility towards one anther, try the intros again in neutral territory.
Use the bonding pouches
they've been in (you can swap those back and forth as well, it shouldn't matter though). Have a neutral bonding
pouch in between. If you don't have three bonding pouches
, no worries, they may all end up in one or the others pouch anyway, as my mine did. This should be done early morning, when their sleepy. Then carry them all together in one bonding
pouch to monitor them for a while, then place them in a cleaned, neutalized cage.
I recommend cleaning the cage real well that you intend to use for the trio (if intro is successful), place neutral fleece (laundered or new) in the cage, and then mix toys
from both cages, if you swapped some, they would be best.
Good luck, if this doesn't work, then try looking for a cage mate.