I'm a newbie too so excuse me if my thought on this isn't accurate. I wouldn't add two more to your one. She might get picked on! You don't say how old she is though. Getting some that are younger than her might work out by twos. I have 9 that are 8 weeks old and when they were 6 weeks old I added a 8 month old to the mix! Newbie move I just plopped her in the middle of the coop in the middle of the day. Well, the pecking order by her commenced immediately. I had to see my babies get "put in their place" one by one. Luckily there were no injuries!
Okay so from the seasoned folks I have learned:
1. Add a new chicken at night when everyone is settled down to roost.
2. Quarantine to make sure you aren't going to make your existing flock sick. I think I've heard keeping the new one separate for 2 weeks is recommended...yeah, I didn't do that either...beginners luck I got no sickness!
I hope you get more replies from people that know more than me!