There are two ways you can achieve this. If you want to keep the offset, change the column that will do the nesting to 12:
<div class="row">
<div class="col-md-12 col-md-offset-3 center">
<div class="row">
<div class="col-md-2 text-center">
<img src="images/nav_benefits.png" width="66" height="65" />
</div>
<div class="col-md-2 text-center">
<img src="images/nav_benefits.png" width="66" height="65" />
</div>
<div class="col-md-2 text-center">
<img src="images/nav_benefits.png" width="66" height="65" />
</div>
</div>
</div>
</div>
Another approach is to place two blank columns on either side.
<div class="row">
<div class="col-md-4"></div>
<div class="col-md-4 text-center">
<div class="row">
<div class="col-md-4 text-center">
<img src="images/nav_benefits.png" width="66" height="65" />
</div>
<div class="col-md-4 text-center">
<img src="images/nav_benefits.png" width="66" height="65" />
</div>
<div class="col-md-4 text-center">
<img src="images/nav_benefits.png" width="66" height="65" />
</div>
</div>
</div>
<div class="col-md-4"></div>
</div>