In this test, I gave a 150px width to all of the boxes (which is 400px less than the parent).The second and third box were given box-flex:1 in addition to their width. The boxes split the extra space equally—each receiving 200px.
1 - Lorem Ipsum only a little bit now. Lorem Ipsum only a little bit now. But add even a slight difference...
2 - Lorem Ipsum only a little bit now. Lorem Ipsum only a little bit more now and then to test.
3 - Lorem Ipsum only a little bit now. Lorem Ipsum only a little bit now. Vary the content be even a tiny bit, and the box with more content (or the first one) will grow larger than the rest.
4 - Lorem Ipsum only a little bit now. Lorem Ipsum only a little bit now.