Do you have a testbench? If not Quartus can automatically write a testbench for you. Just go process/start/start tesbench writer and you will get a template for your testbench. then you have to add the .vht file under simulation under settings. And then open the .vht file and in the section with the comment -- code execute only once. You have to write the input to simulate. In your case you want to simulate buttons being pressed. You have to assign the buttons a value in your case 110. Just write KEY <= "110"; (the same as you would in your VHDL code). Don't forget to assign the clock and reset a value then you just run the simulation.
I hope you understand what i mean :)
