Sometimes it happens that all the small white triangles in the lower left corner for your flyout toolbar disappear. In here you find an explanation to resolve this problem.
Go in your menu to Tools > Toolbar Layout.... In the menu of the pop-up window go to File > Close.
Choose No when asked to save the changes.
Open default.tb in the Toolbar menu via File > Open.... This file is located in C:\Program Files\Rhinoceros 3.0\System.