To achieve this in pure css, here are two options.
1 As mentioned by @xec in comments, assign an arbitrary common height to all the elements
2 Since you already have established fixed widths, you can use a combination of media queries and nth-child selectors to specify a clearing pattern for various widths. For example:
@media (max-width: 680px) {
label:nth-child(3n) { clear: left; }
}