Law of 3:
3x Magic melee damage (princesses, not sure if others ever go into this category)
3x Magic ranged (witch/mage ect)
3x Physical melee damage (HA, Valk, Soldier, Rogue ect)
3x Physical ranged damage (Archer, Pirate ect)
3x Support (Healers, Dancers ect)
Thetis is in the physical column. Role of the unit doesn't play a part, I don't differentiate between duelists ect.
@Karsuman,
The purpose of the rule is to try and force some difficulty. Currently there has not been a map that I couldn't beat with it, and I intend to see how far it can go. Outside of very specific maps (like War of Magic) the use of building teams around a map to me detracts from the strategy stand point. To build a team to a map to me in a way is "cheating". My goal is to have a primary team that can take all comers as is. Of course eventually that will become impossible if you want to 3 star everything, but until that point occurs I enjoy the challenge of playing by the restrictions. If the restriction does change later it will likely go to a rule of 5 (melee / ranged /support) as a replacement.
Just my take for a bit of extra effort by doing this, but given my units, I don't think anyone would think it's wasteful, as the challenge can most assuredly be gotten this way.