I am with the Brotherhood and the Minutemen. What will happen if I join the Railroad as well?

I'm more confused than in any game I have ever played. But it's Bethesda.

Anyways, I have no clue what to do. The Minutemen I joined because they are good, it's common knowledge. I joined the Brotherhood because I've seen them in action. I thought with power armor, vertiberds, I would be unstoppable. I already have 4 power armors. I thought that, well, if they made sure tech was kept away from man and used only for peacekeeping they were good.
So before my actually question, just a little side one... They are good right?

But anyways, my real one is I'm a knight of the Brotherhood and a general of the Minutemen. I just got a brain thing from a courser that should get me in to the Institute. But the railroad can only decode it. I'm afraid that they would want me to join. I'm so nervous. I believe in all the causes (beside the institute those guys are assholes.) but joining the railroad means helping free synths. How would the Brotherhood take this?

I'm taking nick Valentine so only he would know. But I really don't wanna screw up now. I'm nervous that the Brotherhood was not a good choice. I wanted all that Tech so bad, that I forgot.

Solution 1:

You can join all the factions without issues. You have to make a decision later, but not yet at this point.

At some point the quests you get will impact other factions, at that point you'll have to be a bit careful. But the initial joining and the early quests for the Railroad don't have any effect on your status with the Brotherhood.

Solution 2:

It is possible to finish the game with the Railroad, Brotherhood of Steel, and Minutemen all allied together.

Here is a video that describes the process: