That's a lot of words to get through but I think I understand what you are wanting to do.
Unfortunately, the set up you describe is one of the hardest ways to keep discus. A heavily stocked planted discus community tank is beautiful but a challenge even for the most experienced discus keepers. It can be done with varying degrees of success, but it's not something a discus beginner should be trying to tackle.
What you have described will work with 6 adult discus. Not 3" juvies. But even with 6 adult discus, I think you are over stocked with all the other fish you want to include in there.
If you have to start with 3" juvies, they need to be grown out for several months by themselves in a smaller grow out tank until they are about a year old. Once they reach full size and maturity, transfer them to the 125 gallon display tank. If you try to grow out 3" juvies in that planted tank set up, they will not grow much and may even develop disease issues.
There are many stickies at the top of the beginner's page covering how to buy and grow out juvies discus. There are threads here if you do a search covering planted discus tanks and how to maintain them. You need to do some reading and research before you start just to learn how to properly grow out 3" discus. I'm not trying to be dismissive, but there is a lot of info that needs to be covered here that can't be done in a single reply.