The purpose of this research is to describe the development process of cooperative play by three-year-old children and to clarify the structure and the turning point of their play. The target cooperative play is “Playing House” in the classroom in which eight children participated. In the analysis, the acts and utterances of the children were transcribed based on the video images. The development process model of cooperative play was created from the transcription by using the Trajectory Equifinality Model (TEM). As a result, the following characteristics became apparent: 1) “Playing House” is composed of three different processes of play (the process of building the structure of the house with blocks, the process of “Party Play,” and the process of participating in a negative way to “Playing House” and playing the family structure). These three processes are transformed and maintained in a different way from other; 2) The division of roles of play occurs because each child pursues a niche for themselves. Eight children had each chosen the participatory method of “Playing House” suitable for their own interests and situation. As a result, the elements required for a “House” were satisfied; 3) The structure of “Playing House” is flexible and weak. “Playing House” allows for the participation of many children and reflects their interests. However, children are also allowed to leave the play, and thus the theme of play easily disappears. Through the analysis above, the structure of the development process through which three-year-old children’s cooperative play is transformed or maintained became clear.