With Gnome 3, the minimize and maximize buttons have been removed. There is only a close button. If you want to add them back, you can do so with one of the following methods.
Open the gconf configuration tool by pressing Alt + F2 and running:
Go to Desktop > Gnome > Shell > Windows
Double click :close next to button_layout to edit.
2. Gnome Tweak Tool
The super easy way to do it, is with the Gnome Tweak Tool. Under Shell, change the “arrangement of buttons on the titlebar” to “All”.