Which of these configurations would require a 3" trap?

The requirement for a 3" trap is primarily connected to the flow rate and the volume of waste generated by a fixture. High-flow water closets, which are designed to accommodate increased water usage typically used in residential and commercial settings, necessitate a larger trap to effectively handle the greater volume of wastewater and prevent clogging.

A 3" trap is essential for high-flow fixtures because it can manage more substantial sewage flow and provides a better seal against sewer gas. The larger diameter facilitates better drainage and minimizes the risk of backflow, which can occur if the plumbing system can’t handle the discharge rate adequately.

In contrast, other fixtures listed, such as urinating fixtures, low-flow water closets, and standard urinals, do not require such a large trap as their flow rates and waste volumes are significantly lower. Consequently, these fixtures can be adequately serviced with smaller traps, reducing the risk of blockages while complying with plumbing codes.