Bootstrap 3 officially dropped support for IE7, but you can try a polyfill library such as: https://github.com/coliff/bootstrap-ie7 if you really need support for such an old browser (such as in a corporate environment).
IE8 is currently supported (but will be dropped in Bootstrap v4). In order to get IE8 to work properly, you will need to add respond.js to your app.
<!-- HTML5 Shim and Respond.js IE8 support of HTML5 elements and media queries -->
<!-- WARNING: Respond.js doesn't work if you view the page via file:// -->
<!--[if lt IE 9]>
<script src="https://oss.maxcdn.com/html5shiv/3.7.2/html5shiv.min.js"></script>
<script src="https://oss.maxcdn.com/respond/1.4.2/respond.min.js"></script>
<![endif]-->
