Add below css in your theme style.css file, change font size if you need
@media screen and (max-width: 56.875em) {
.main-navigation li {
float: left;
width: 33.333333%;
text-align: center;
font-size: 12px;
}
}
UPDATE:
To make menu items inline apply floats on list elements not on nav tag
I checked using inspect elements and code i sent works fine. check that rule does not override by any other CSS in hierarchy.