You have alot of mistakes in your HTML, even if you follow the suggested answer in comments with your current HTML no way you can make it work
Correct HTML
<div class="row">
<div class="col-md-4">
<div class="panel panel-default" style="height:350px;width:400px;">
<div class="panel-heading"> 1</div>
<div class="panel-body">
<img src="mb3.jpg" alt="cannot load image" `enter code here`class="img-responsive" />
<button type="button" class="btn btn-primary btn-block">button</button>
</div>
</div>
</div>
<div class="col-md-4">
<div class="panel panel-default" style="height:350px;width:400px;">
<div class="panel-heading"> 2</div>
<div class="panel-body">
<img src="mb3.jpg" alt="cannot load image" `enter code here`class="img-responsive" />
<button type="button" class="btn btn-primary btn-block">button</button>
</div>
</div>
</div>
<div class="col-md-4">
<div class="panel panel-default" style="height:350px;width:400px;">
<div class="panel-heading"> 3</div>
<div class="panel-body">
<img src="mb3.jpg" alt="cannot load image" `enter code here`class="img-responsive" />
<button type="button" class="btn btn-primary btn-block">button</button>
</div>
</div>
</div>
</div>
Mistakes in your HTML
row
inside panel
and repeating <div class="panel-body">
- This is not a class
<div col="col-md-4" >
should be <div class="col-md-4" >
- unnecessary
<br>
tags
Right way to do it
First you need to understand how Bootstrap Grid system works
BootStrap Grid system
<div class="row">
<div class="col-md-4">
<div class="panel">
<div class="panel-heading"></div>
<div class="panel-body">
//Your content goes here
</div>
</div>
</div>
<div class="col-md-4">
<div class="panel">
<div class="panel-heading"></div>
<div class="panel-body">
//Your content goes here
</div>
</div>
</div>
<div class="col-md-4">
<div class="panel">
<div class="panel-heading"></div>
<div class="panel-body">
//Your content goes here
</div>
</div>
</div>
</div>
Fiddle Example