It really depends. For attack, it depends on your opponent, choose heroes which are strong against them. If you attack a Boril or Cyprian for example, you want to bring a dispeller.
As you build your team and get more heroes, you will be able to try different combinations and tactics. Threads like this are a good starting point.
If you are choosing a defense, you usually want to pick one of each color.
But if you only started playing 3 weeks ago, you should probably start with your 3* heroes first. I doubt you will already have the materials to fully ascend your 4s, or even your 5s.
