With Bootstrap 4.1 I found a width solution by percentage:
The following shows an input-group with 4 Elements: 1 text an 3 selects - working well:
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css" integrity="sha384-MCw98/SFnGE8fJT3GXwEOngsV7Zt27NXFoaoApmYm81iuXoPkFOJwJ8ERdknLPMO" crossorigin="anonymous">_x000D_
_x000D_
<div class="input-group input-group-sm">_x000D_
<div class="input-group-prepend">_x000D_
<div class="input-group-text">TEXT:</div>_x000D_
</div>_x000D_
<select name="name1" id="name1" size="1" style="width:4%;" class="form-control">_x000D_
<option value="">option</option>_x000D_
<!-- snip -->_x000D_
</select> _x000D_
<select name="name2" id="name2" size="1" style="width:60%;" class="form-control">_x000D_
<option value="">option</option>_x000D_
<!-- snip -->_x000D_
</select> _x000D_
<select name="name3" id="name3" size="1" style="width:25%;" class="form-control">_x000D_
<option value="">option</option>_x000D_
<!-- snip -->_x000D_
</select>_x000D_
</div>
_x000D_