Sure it's possible.

You are half way through this. Your problem is, that inside a button group, they have a selector for not(:first-child) and not(:last-child) . You need to overwrite this one, too.

.btn-group>.btn:not(:first-child):not(:last-child) { border-radius: 25px }

You should probably be applying the border-radius on each individual button instead of on the whole btn-group.

<div class="btn-group" role="group" aria-label="...">
    <button type="button" class="btn btn-default btn-rounded">Left</button>
    <button type="button" class="btn btn-default btn-rounded">Middle</button>
    <button type="button" class="btn btn-default btn-rounded">Right</button>
</div>

...and then the CSS class

.btn-rounded {
    border-radius: 25px;
}