Helping two budgies get along

[Sim]

Hello everyone and Happy New year! 

Like I said in my introduction thread, sooner or later I was going to tell you something more about the complicated story of my flock.

Well, those of you who replied to my other thread know that two of my budgies, Charlie and Sandy, are a couple, while the third one, the elderly Sid, lives alone. It wasn't always like this though.
Sid was my first budgie and he had some chicks with his (now late) "wife". I kept one of the chicks, an unfortunately disabled female, and when she was an adult I bought Charlie to give her a mate. Later on, one year after Sid had become a widower, I got Sandy from a friend to keep him company. The two pairs were in two different cages, but next to each other.

Problems arised when Charlie became very insistent with Sid's daughter and even violent (he plucked out her lower back feathers) because they couldn't mate properly due to her disability (and I guess he knew she had something wrong in general). At the same time, he became interested in Sandy and developed an obsession towards her.
I tried dividing him and taking him away temporarily, but his behavior wouldn't change. In the end, the only thing that worked was to switch the pairs: Charlie with Sandy, and Sid with his daughter. This arrangement only lasted a month, as Sid's daughter died in October because of other pre-existing health-issues.

That's how I ended up in the current situation. Even though I read that usually "two's a company, three's a crowd" for budgies, I was hoping it could be possible for Sid to live in the same cage as Charlie and Sandy.
The thing is, when Charlie was obsessed with Sandy he developed a rivalry with Sid, and the two even fought (no serious injuries, of course) on the few occasions I allowed them to meet each other directly.
Now, Sid doesn't seem to be interested in catching Sandy's attention again, but he fears/despises Charlie: when they meet outside of the cages, he flies away if Charlie gets close. On the other hand, I still don't know if Charlie still feels a rivalry with him or just wants to socialize.

Do you think there is a way to improve their relationship and maybe someday house them together? Or should I just leave things as they are?
Thanks in advance! And I'm sorry for the long message. 

[Hezz]

I would leave things as they are for the time being and allow them all to have time together outside the cage. Sometimes the cage can become a battleground simply due to the fact that the birds are confined. Normally I would say all should be fine in together but with the past history and also given Sid’s age, I would give them time together but without the restrictions that a cage has. Sid doesn’t deserve to be hassled by Charlie (if Charlie feels that way inclined) but he also deserves to feel safe and comfortable and in the cage with Charlie might just be too confining for him. Watch them for a while when they are out together and see how the two boys are getting on. Leave the cage doors open when they are out as well and see who ends up in what cage.

[Sim]

Thank you!
I left both the cages open most of the time in the last two days, but I still haven't understood whether Charlie goes near Sid just out of curiosity/as a friendship attempt or as an act of aggression. Maybe I'll post a video if I manage to record one of their interactions.
Regardless of what Charlie's intentions are, Sid flies away basically every time he's approached by him. Plus, Charlie likes to go into Sid's cage, and when it happens Sid gets out immediately. I'm keeping a close eye on him to make sure he doesn't get too tired or stressed (I even locked him inside alone when Charlie was getting too invasive).
I also had the idea to offer them a millet spike from my hand. If they eat it at opposite ends, they can focus on something else - while being relatively close - and hopefully calm down and get used to each other.

[Hezz]

Good idea with the millet, and hope that works for poor Sid. It won’t hurt either if Charlie is shut in while Sid is having free-flying time, with or without the company of Sandy. If Sandy is out with Sid, that might be too much for Charlie to be happy with but on the other hand Sid does deserve to have a chat with another bird sometimes.

It may take some time before you notice any real change if it is going to happen but a little bribery in the way of millet for both is perfectly acceptable.