Actually LOC and PML cones fit 3" I.D. tubes. Their outer diameter is ~3.1". I would expect a Performance Rocketry cone to be the same as these body tube sizes are pretty much universal. Some fitment problems every so often but for the most part they are all interchangeable.
Drt_trkr: Are you using a commercial tube? If so, whose? Also if you are using a 3" tube from the post office I know personally that they fit 3" PML nose cones.