Дмитрий

style.css :
.navbar-nav {
float: none;
text-align: center;
}
.navbar-nav li {
float: none;
}

.main {
  width:700px;
	height : 400px;
  margin:0 auto;
  background:red;
  text-align:center;
	display: flex ;
	flex-wrap: wrap;
	justify-content: center;
}

.item {
	flex:1;
  background:green;
  width:200px;
	height: 100px;
  color:#fff;
  font-size:16px;
}

table 
{
    table-layout: fixed;
    width: 100%;
    white-space: nowrap;

}
.th-1column {
    width:10%;
}
.th-2column {
    width:20%;
}
.th-3column {
    width:70%;
}

<table>
     <thead>
        <tr> 
               <th class="th-1column">1 столбец</th>
               <th class="th-2column">2 столбец</th>
               <th class="th-3column">3 столбец</th>
        </tr>
         <tbody>
                  <tr>
                        <td>Вася</td>
                        <td>Пупкин</td>
                        <td>go@to.ass</td> 
                 </tr>
          </tbody>
    </thead>
  </table>