Use before & after Pseudo-element to make a line

You don't need both :before and :after, either of the 2 will be enough and as you've been told, you don't need an image. See the approach below. 

#header {
    width: 100%;
    height: 50px;
    margin: 50px 0;
    text-align: center;
    font-size: 28px;
    position: relative;
    background-color: #57585C;
}

#header:after {
    content: '';
    width: 100%;
    border-bottom: solid 1px #fff;
    position: absolute;
    left: 0;
    top: 50%;
    z-index: 1;
}

h3 {
    background-color: #57585C; /* Same as the parents Background */
    width: auto;
    display: inline-block;
    z-index: 3;
    padding: 0 20px 0 20px;
    color: white;
    position: relative;
    font-family: calibri;
    font-weight: lighter;
    margin: 0;
}
<div id="header">
   <h3>Last Projects</h3>
</div>

In case you need <h3> title to have transparent background - you can use both :before and :after and display: flex

More about flex-grow you can read here https://developer.mozilla.org/en-US/docs/Web/CSS/flex.

body {
  background: linear-gradient(0.25turn, #3f87a6, #000000, #f69d3c); /* example of custom background */
}

#header {
  display: flex;
  flex-wrap: nowrap;
  justify-content: flex-start;
  align-items: center; /* making vertical centerign of all children */
}

#header::before, #header::after {
  content: '';
  flex: 1 1 auto; /* the first digint is 'flex-grow: 1', helps elemet to occupy all free space */ 
  border-bottom: solid 1px #fff;
}

h3 {
  flex: 0 1 auto; /* the first digint is flex-grow: 0 */ 
  padding: 0 15px 0 15px;
  color: #fff;
}
<div id="header">
  <h3>Last Projects</h3>
</div>

Tags:

Html

Css

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