Middle box centered variable width, left and right float directly attached

Question

I\'m designing my own Anki-Flashcards for learning Japanese:

I have three boxes of variable size in a line. The middle one should be centered and has a variable width. T

Answer 1

.right {
  float:left;
  border: 1px solid;
}
.middle {
  margin-left:3px;
  float:left;
  border: 1px solid;
}