These are heroes that you can pick pretty much whenever in the draft phase, the earlier the better though. They're objectively the strongest heroes provided that they are played adequately and will increase your chances of winning.
These heroes are still pretty strong all around the board, just not quite as strong as those on the Tier 1 list. Some of these heroes are better than others depending on the situation, but are usually pretty self sufficient and have their own ways they can impact the game.
Some of these heroes are still pretty strong situationally, but those situations are a little bit more rare than the ones on the tier 2 list, landing them a spot in tier 3. Some are also still able to make a high impact on the game.
For example, picking Cassia is very strong into an enemy team consisting of all auto attack damage (Valla, Greymane, Illidan, etc.), because her passive lets her thrive in those scenarios, as opposed to if she was versus a Gul'Dan, or a Li-Ming, she would be a very weak pick.
These heroes are usually not so great. Maybe occasionally you can get some value out of some of them, but I would suggest to usually avoid these heroes.
You should probably just not pick these in hero league unless your name is Glogan and you're a Gazlowe god or you're Zugrug and you have an insane winrate with Murky. I would recommend focusing on other heroes.