How do I set responsive span width based on browser size

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bootstrap

I have this code that is for desktop only.

<div class="span3 dashboard-blocks">
           <h3>Current Employees </h3>
           <b style="font-size:16px;">(@Model.EmployeeCount)</b>&nbsp;&nbsp;|
           @Html.ActionLink("View", "List", "Employee")<br>
           <br>
       </div>

I want to show span6 for as class for mobile but span3 for larger screen size.
How can i do that please help me thanks...

What I have tried:

<div class="span3 dashboard-blocks">
            <h3>Current Employees </h3>
            <b style="font-size:16px;">(@Model.EmployeeCount)</b>  |
            @Html.ActionLink("View", "List", "Employee")<br>
            <br>
        </br></br></div>