Dozo

From BoardGameGeek:

Dozo is one of Stancraft's line of puzzles and abstract games under the "Hoyle Official Games" line (the company may or may not be the same one called "Hoyle Products" a few years later). Dozo consists of a board with 28 spaces in an equilateral triangle and 7 Go stones in each of 4 colors. On a turn, a player places any color stone on the board. The object is to be the first player to place a piece which forms the third vertex of an equilateral triangle with 2 other pieces of the same color.