/* rules for phones */
@media only screen and (max-width:720px)
{
    .tablet {display:none;}
    .desktop {display:none;}
}
/* CSS rules for tablets */
@media only screen and (min-width:721px) and (max-width:960px)
{
    .desktop {display:none;}
    .phone {display:none;}
}
/* CSS rules for desktops */
@media only screen and (min-width:961px)
{
    .tablet {display:none;}
    .phone {display:none;}
}