Can a poly relationship have one person who is dating 2 people but the 2 people aren’t dating each other, say person A is dating person B and person C but B isn’t dating C. is that considered a good/healthy poly relationship?

Yes. That is called V or Vee shaped polyamory. 

There is no canonical list somewhere of what is “considered” good or healthy. Some configurations work for some people but would be unhealthy and unfulfilling for others. Don’t let anyone tell you that your way of living and loving needs to be validated by anyone else. If it’s working for you, it’s working for you.