Flexbox: 3 divs, 2 columns

I'm new to flexbox, so please bear with me. My html:

<div class="container">
    <div class="one"></div>
    <div class="two"></div>
    <div class="three"></div>
</div>

With flexbox, I am trying to achieve the following layout:

.

What's the simplest way I can achieve this without setting a height for the container? .one is expected to change in height.