I have just copy paste your code in my html partial view but checkbox is coming normal not as shown in your site.

<div class="form-row">
<div class="col-md-12 form-inline">
<div class="form-check">
@Html.CheckBoxFor(model => model.IsEmployee, new { @class = "form-check-input", @checked = "checked", @id = "chkIsEmployee" })
<label class="form-check-label" for="chkIsEmployee">Is Employee</label>
</div>

<div class="form-check">
@Html.CheckBoxFor(model => model.IsActive, new { @class = "form-check-input", @checked = "checked", @id = "chkIsActive" })
<label class="form-check-label" for="chkIsActive">Is Active</label>
</div>

<div class="form-check">
@Html.CheckBoxFor(model => model.IsAdmin, new { @class = "form-check-input", @checked = "checked", @id = "chkIsAdmin" })
<label class="form-check-label" for="chkIsAdmin">Is Admin</label>
</div>
</div>
</div>