Crit and mana troops are better for defense, that’s why they have earned more stars (they are more valuable)
But what is the best use for 3* troops, I have been thinking this for a while. They are not useless.
For offence very fast, average, slow and very slow needs a mana troop, so only choice is that they are made for fast heroes (advantage is so small). You just have to find the most suitable one. That would also help you to choose nodes correctly, if you know what troops you are using. Also I don’t recommend you to feed secondary crit troop right away