How to use all of my core i3 procecors

How many i3 processors do you have? Or are you refering to all of the cores? If the latter and are using Win7, press CTRL-ALT-DEL and go to Task Manager/Performance. You will be shown a graph of cores.

Once you see the graph start running some programs.

If you have hyperthreading turned on in the bios you will see 4 cores, just remember two of them are hyperthreads and not true cores, otherwise you will only see 2 cores.