Took a decent amount of tinkering, but I came up with something workable based around absolute positioning, I did not apply any hover actions, those should be relatively easy for you to do (don’t forget to z-index). Here’s the CSS I would use:
#m {
list-style: none;
}
#m > li {
padding: 15px;
float: left;
}
#m .sub-menu {
display: none;
position: absolute;
top: 50px;
left: 0;
width: 300px;
}
#m .sub-menu .sub-menu {
position: static;
}
#m .sub-menu .sub-menu .sub-menu {
position: absolute;
width: 200px;
top: 0;
left: 300px;
}
#m .current-menu-ancestor > .sub-menu {
display: block;
}
#m .current-menu-parent > .sub-menu {
display: block;
}
#m
is your menu holder, you can set that to whatever, the rest is default from wordpress, so you can’t change that around too much without going to a custom walker or something…but why would you do that? Anyways, if you need help with the hover actions or anything, let me know.