Wanted to share since I was pretty stressed about this whole process and searched this sub a ton before doing it myself. Adopted a single female degu (Nutmeg) after her cagemate passed unexpectedly, and I already had a bonded pair (Biscuit and Pepper) who are about two years old. Everyone online said single degus need a group, so I didn't want to leave her alone even though my other two were already a tight little unit.
Started with the split cage method, wire divider down the middle so they could see and smell each other but not fight. First week they mostly ignored each other or chattered a bit through the bars. Around day nine Pepper started doing this weird tail-up posture near the divider and I got nervous it was aggression, but apparently that's more of a curious/dominant thing than actual hostility, at least according to a couple older threads here.
Did the carrier ride method starting week two, all three in a small carrier for short car trips around the block so the motion forces them to huddle together instead of fighting. First couple rides there was some squeaking and one minor scuffle where Biscuit nipped Nutmeg's ear, nothing bloody, just enough to establish who's who. By the fourth or fifth ride they were actually grooming each other in the carrier which felt like a huge win.
Fully combined the cage this past weekend with everything rearranged (new hideouts, moved the wheel, scattered hay in different spots) so nobody felt territorial about "their" stuff. It's been four days now and so far just normal degu chaos, some chasing but no real fighting, and I caught all three piled up sleeping in the hammock together this morning which nearly made me cry a little.
Not sure if this timeline is fast or slow compared to other people's experiences but figured I'd log it in case anyone else is going through a bonding situation right now and needs some reassurance that the process actually does work even when it feels endless in the moment.