The name is a reference to the popular game Tic-Tac-Toe. However, the goal is the opposite: You're avoiding making three-in-a-row sets.
Puzzle and Goal
An unsolved puzzle consists of a square grid. Some squares have x or o in them.
The goal is to fill each cell with an x or an o.
The solved grid must satisfy the following conditions:
Each rank must contain an equal number of xs and os.
There can be no set of three xs or os in a rank; however, sets of three or more or allowed in diagonals.
Each row and each column has a unique pattern of xs and os; however, a pattern can appear in a row and a column.