What is your role on the three projects?
(Click the tab menu on each, then from the Project menu, point to Project settings, and select User permissions).
If a Users Setup dialog (with four questions) appears, you are the administrator on the project.
Another possibility for the project you cannot edit is that the project does NOT have Editing enabled.
(Click the tab menu on the one that is blocked, then from the Project menu, point to Project settings, and select Project properties, On the Advanced tab, see if the check box to the left of “Editing Enabled” has a tick mark.)