@Girigiri
Just before the last mission, you can choose to marry anyone in your army who is:
1)Alive
2)Has an A support with you and the required points for S Support.
3)Has a compatible sexual orientation.
The bit about "Edelgard and 2 dudes" is probably referring to the house leaders. At the start of the game, you have to choose to teach one of the three houses- Edelgard's Black Eagles house, Dimitri's Blue Lions house or Claude's Golden Deer house- and while the house leader is implied to have feelings for Byleth, you can choose whoever meets the above three criteria.
As for how you get together with Rhea, that's something I can't explain without spoiling the plot, but it requires taking a specific story route, making the right decision when the time comes and getting to an A support with Rhea before the end of Part 1.