Layout with 3 columns of different sizes using Flex : <p>I have a section of the site that must contain three elements side by side. Each element has a minimum width. If the width of the screen is not sufficient to contain all three elements, those that do not fit, go into a new line.</p>

<p>To build this layout I used <a href="https://css-tricks.com/snippets/css/a-guide-to-flexbox/" rel="noreferrer">Flex</a>. 
<a href="https://plnkr.co/edit/jAtSd1aCn4Cz6B0VOXa6?p=preview" rel="noreferrer"><strong>Code</strong></a>.</p>

<p>html:</p>

<pre><code>&lt;main id='main'&gt;
    &lt;div id='firstRow' class='row'&gt;
        &lt;div id='col1C' class='col'&gt;col1C title
            &lt;div id='col1Ccon'&gt;col1Ccon content&lt;/div&gt;
        &lt;/div&gt;
        &lt;div id='col2C' class='col'&gt;col2C title
            &lt;div id='col2Ccon'&gt;col2Ccon content&lt;/div&gt;
        &lt;/div&gt;
        &lt;div id='col3C' class='col'&gt;col3C title
            &lt;div id='col3Ccon'&gt;col3Ccon content&lt;/div&gt;
        &lt;/div&gt;
    &lt;/div&gt;
&lt;/main&gt;
</code></pre>

<p>css:</p>

<pre><code>:root {
    --w1Col: 478px;
    --w2Col: 370px;
    --w3Col: 350px;
    --wSum3: calc(var(--w1Col) + var(--w2Col) + var(--w3Col));
}

html {
    /*height: 100%;*/
}

body {
    margin: 0;
    font-size: 9pt;
    font-family: 'Roboto', sans-serif;
    font-weight: 300;
    background-color: whitesmoke;
    height: 100%;
    display: flex;
    flex-direction: column;
}

/***************************************************************
 * Layout first row
 */
#main {
    background-color: white;
    /*border: 4px solid red;*/
    color: black;
    height: 100%;
    flex-grow: 1; /* ability for a flex item to grow if necessary */
    flex-shrink: 0; /* ability for a flex item to shrink if necessary */
    flex-basis: auto; /* defines the default size of an element before the remaining space is distributed */
}

#firstRow {
    background-color: white;
    /*border: 1px solid black;*/
    margin: 10px;
    padding: 10px;
    display: flex;
    flex-direction: row; /* colums layout */
    flex-wrap: wrap; /* wrap in multiple lines if it's necessary */
    /*min-width: var(--wSum3);*/
    /*justify-content: flex-end;  defines the alignment along the main axis */
}

#firstRow .col {
    /*border: 1px solid black;*/
    flex: 1;
    padding: 5px;
    text-align: center;
}

#col1C {
    background-color: green;
    width: var(--w1Col);
    min-width: var(--w1Col);
    order: 1; /* column order */
    flex-basis: 40%;/*var(--w1Col);  column width */
    justify-content: flex-end;

}
#col2C {
    background-color: blue;
    width: var(--w2Col);
    min-width: var(--w2Col);
    order: 2; /* column order */
    flex-basis: 35%; /* var(--w2Col);  column width */
    justify-content: flex-end;
}
#col3C {
    background-color:  red;
    width: var(--w3Col);
    min-width: var(--w3Col);
    order: 3; /* column order */
    flex-basis: 25%; /*var(--w3Col);  column width */
    justify-content: flex-end;
}

#col1Ccon, #col2Ccon, #col3Ccon {
    /*border: 1px solid black;*/
    margin: 0 auto; /* center content */
}

#col1Ccon {
    width: var(--w1Col);
    background-color: lightgreen;
    height: 200px;
}
#col2Ccon {
    width: var(--w2Col);
    background-color: lightblue;
    height: 150px;
}
#col3Ccon {
    width: var(--w3Col);
    background-color: salmon;
    height: 200px;
}
</code></pre>

<p>The code works but there are some tricks I would like to fix.</p>

<ol>
<li><p>the 3 columns all have the same width and I would like to be able to choose this width. This is because the first element has a minimum width greater than the other two, so the edge is smaller and aesthetically ugly. See <a href="https://jsfiddle.net/6rys2xe5/4/" rel="noreferrer">the same example</a>, I changed only colors</p></li>
<li><p>the remaining space now grows on both sides of the <code>col*Ccon</code> containers. Instead I would like to grow only on the left side of <code>col1Ccon</code> and on the right side <code>col3Ccon</code>. I would therefore like the contents of the site (<code>col1Ccon + col2Ccon + col3Ccon</code>) to always remain in the center of the page and what changes are the "border" that grow and decrease.</p></li>
</ol>

<p>I'm stuck and any suggestions are greatly appreciated. Thanks :)</p>
