There is only one file type that uses the .P3 extension:
Primavera P3 Project File
Project scheduling file created by Primavera P3, a project planning software often used for construction projects; saves the tasks, timeline, and resources for completing a project; allows the project manager to plan and track progress.
Primavera P3 was developed by Primavera Systems, who was acquired by Oracle in 2006. Primavera P6, Oracle's continuation of the Primavera software, uses the .XER extension for projects. P3 projects can still be opened in Primavera P6 by choosingfrom the application menu.
NOTE: Oracle discontinued support for Primavera P3 on December 31, 2010.
Updated: December 3, 2011