How to join 3 traces is a PCB?
The 90º intersection will not be a issue for most traces. If your trace is very thin (&lt; 15 mill) and/or the PCB boards aren't being professionally manufactured you can use mitered traces (chamfer) as helloworld922 pointed out. Adding chamfers will eliminate the 90º intersection and help to strengthen the trace. For high frequency traces (> 1GHz) it also helps reduce signal reflections.




  The lines have constant impedance along their length regardless of
  routing which is a highly desirable property. Note: this is less true
  at the highest frequencies where abrupt corners in a track also cause
  reflections. For this reason most PCB designs employ mitred (45°) or
  chamfered (curved) corners.



Best practice in circuit board design


There are other methods to help prevent signal reflections, like the following image shows:



The image above is from Microstrip, Stripline, and CPW Design.

For more reading material, take a look at 90 Degree Corners: The Final Turn.