Answer:
 1/6
Step-by-step explanation:
Let x represent the proportion of time the third person spends on the project. You want ...
 1/2 + 1/3 + x = 1 . . . . full-time equivalents
 5/6 + x = 1 . . . . simplify
 x = 1/6 . . . . . . . subtract 5/6
The third person should budget 1/6 of their time to the project.