SO http://is.gd/STqInA

HTML

<link rel="stylesheet" href="http://netdna.bootstrapcdn.com/bootstrap/3.0.0/css/bootstrap.min.css">
<form>
    <div class="form-group input-group">
        <span class="input-group-addon">X</span>
        <input type="text" class="form-control" id="svX">
        <span style="width: 55px;" class="input-group-addon">km</span>
    </div>
    <div class="form-group input-group">
        <span style="width: 55px;" class="input-group-addon">X</span>
        <input type="text" class="form-control" id="svX">
        <span class="input-group-addon">km</span>
    </div>
    <div class="form-group input-group svX3">
        <span style="width: 55px;" class="input-group-addon">Xxxxxxxxxx</span>
        <input type="text" class="form-control" id="svX">
        <span style="width: 55px;" class="input-group-addon">km</span>
    </div>
</form>

CSS

.svX3{
    width:100%;
}