In the two player map, 3/4 of the entrances to each player's sector lead to the other player's sector. That's balanced.
In the four player map, 3/4 of the entrances to each player's sector lead to another player's sector. Each player has one entrance into each other player's sector. That's balanced.
In the three player map, as shown in the rulebook on page 5, the yellow player has only 2/4 of the entrances to his sector lead to another player's sector. The yellow player has one entrance to the blue player's sector and one entrance to the red player's sector. The blue player has 3/4 of his entrances lead to another player's sector. He can move directly to the yellow player's sector, or he can use the purple portals AND the blue portals to enter the red player's sector. The red player is in the same situation as the blue player.
This doesn't make any sense. Why should the yellow player be at a disadvantage in terms of movement? The easy way to balance this setup is to remove the blue portals. Then it's balanced. Each player can visit either of the other two players, and it's all through entrances that are next to each other. The blue portals stick out like sore thumbs on this setup.