There are 3 win conditions (paraphrasing from link below) :
- usual tic-tac-toe format: the same sized pieces in a row [horizontal, vertical, diagonal]
- three pieces in a row [horizontal,vertical,diagonal] in either ascending or descending size order [e.g. small, medium, then large going from top left to bottom left]
- three concentric pieces in the same space [i.e. in the same space you have your small, medium, big]
Even though this is more complicated than tic-tac-toe it seems like the game tree is not so expansive and it should be fairly easily solvable with e.g. alpha-beta pruning? So then what is the optimal strategy if you are in a 2 player game, or if you are in the full 3-4 player game?
See this link for more: http://www.marblesthebrainstore.com/otrio.htm