Why is it that when I go into a friend safari, sometimes there will be three poke ball symbols when I see the Pokemon that can be caught, but sometimes there are only two?
Well, the 3 Pokeballs mean that there are 3 Pokemon that you can catch but the third Pokemon is usually rarer than the other 2.
Hope I helped!
Those are the available Pokemon in that friend safari. They will only have three Pokemon in it if the person has beaten the elite four and connected to the internet sometime after that(or maybe connected to the internet as the same time you have, i'm not 100% sure on the requirement).