1.Boone - Novac
You can find him inside of the dino's mouth guarding the town of Novac after 9PM. You will need to complete the side quest "One For My Baby" that he gives you by talking to him in order for you to be able to recruit him to follow you.
2.ED-E - Primm
ED-E is a robotic companion that you can find in the town of Primm. It will be on the table inside of the Nash Residence. Once you repair him, his Side Quest "ED-E My Love" will become available.
3.Veronica - 188 Trading post
You can find Veronica at the 188 Trading Post, north of Vault 11 and El Dorado Dry Lake.You will just need to finish your discussion positively with her including positive things in relation to when she is asking about you about the Brotherhood of Steel.