How can I show three columns per row?

I have been trying to show three columns per row. Is it possible using flexbox?

My current CSS is something like this:

.mainDiv {
    display: flex;
    margin-left: 221px;
    margin-top: 43px;
}

This code puts all content in a single row. I want to add a constraint to just shows three records per row.

This may be what you are looking for:

http://jsfiddle.net/L4L67/

body>div {
  background: #aaa;
  display: flex;
  flex-wrap: wrap;
}

body>div>div {
  flex-grow: 1;
  width: 33%;
  height: 100px;
}

body>div>div:nth-child(even) {
  background: #23a;
}

body>div>div:nth-child(odd) {
  background: #49b;
}

<div>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
</div>

Even though the above answer appears to be correct, I wanted to add a (hopefully) more readable example that also stays in 3 columns form at different widths:

.flex-row-container {
    background: #aaa;
    display: flex;
    flex-wrap: wrap;
    align-items: center;
    justify-content: center;
}
.flex-row-container > .flex-row-item {
    flex: 1 1 30%; /*grow | shrink | basis */
    height: 100px;
}

.flex-row-item {
  background-color: #fff4e6;
  border: 1px solid #f76707;
}

<div class="flex-row-container">
  <div class="flex-row-item">1</div>
  <div class="flex-row-item">2</div>
  <div class="flex-row-item">3</div>
  <div class="flex-row-item">4</div>
  <div class="flex-row-item">5</div>
  <div class="flex-row-item">6</div>
</div>

Hope this helps someone else.

Try this one using Grid Layout:

.grid-container {
  display: grid;
  grid-template-columns: auto auto auto;
  padding: 10px;
}
.grid-item {
  background-color: rgba(255, 255, 255, 0.8);
  border: 1px solid rgba(0, 0, 0, 0.8);
  padding: 20px;
  font-size: 30px;
  text-align: center;
}

<div class="grid-container">
  <div class="grid-item">1</div>
  <div class="grid-item">2</div>
  <div class="grid-item">3</div>  
  <div class="grid-item">4</div>
  <div class="grid-item">5</div>
  <div class="grid-item">6</div>  
  <div class="grid-item">7</div>
  <div class="grid-item">8</div>
  <div class="grid-item">9</div>  
</div>

You can now use grid-auto-flow

https://jsfiddle.net/chalcrow/bqye79kr/1/

CSS

.grid-flow {
	display: grid;
	grid-auto-flow: row;
	grid-template-columns: repeat(3, 1fr);
	grid-template-rows: repeat(2, 1fr);
}

Note - the repeat(3, 1fr) means '3 columns, each taking up 1 (equal) fraction of the available space.

HTML

<div class="grid-flow">
    <div>1</div>
    <div>2</div>
    <div>3</div>
    <div>4</div>
    <div>5</div>
    <div>6</div>
</div>

Result