Aligning div and section elements with flex CSS - html

Question

I'm currently redoing some code to update to HTML5 (and trying to de-uglify CSS), but so far my alignment and layout doesn't seem to be working as expected. Here's part of what I have:
<main class="width">
<div class="r1">
<section class="c1">
<h1>header</h1>
stuff
</section>
<!--Line Break-->
<section class="c1">
<h1>header</h1>
stuff
</section>
</div>
<!--Next Column-->
<div class="r2">
<section class="c2">
<h1>header</h1>
stuff
</section>
</div>
</main>
CSS:
#font-face {
font-family:'Roboto';
font-style:normal;
font-weight:400;
src:url(/resources/font.woff) format('woff');
}
* {
margin:0;
padding:0;
font-size:small;
font-family:Roboto;
vertical-align:middle;
border:none;
text-decoration:none;
}
.width {
margin:0 auto;
width:86%;
min-width:1000px;
}
.bg {
min-width:1000px;
background:linear-gradient(#444, #000, #444);
}
main {
line-height:1.5;
text-align:center;
}
body>.main {
font-size:0;
}
section {
border:1px solid #BBB;
background:#000;
border-radius:7px;
display:inline-block;
overflow:hidden;
}
h1 {
background:linear-gradient(#444, #000, #444);
padding:5px;
color:#FFF;
}
.r1 {
color:#FFF;
display:inline-block;
}
.r2 {
color:#FFF;
display:inline-block;
}
.c1 {
width:33%;
}
.c2 {
width:66%;
}
(Ignore r1 and c1 naming. They don't actually represent a row or column)
However, this does not work the way it used to. What I'm trying to do is to have two rows of 66% width columns above each other, and then next to them, a larger single 33% width column to the right of them. Instead, I've got the the first two sections on one line, taking up a total of 66% width (so each section is 33% width), and then the next section ends up underneath it.
Ideally, I want to use the flex code, so that I can use up all available space, but I'm still new to it, and don't know my way around it very well. The problem with using percentage widths is that even after white-space removal, things still break to the next line when zoomed in enough, and I think that the flex method is more elegant and modern anyway.