Вы можете использовать CSS flex-box для достижения своей цели.
* {
margin: 0;
padding: 0;
}
body {
background-size: cover;
}
.undefinedheader {
width: 40%;
height: 30px;
background-color: grey;
margin: auto;
margin-top: 60px;
box-shadow: 0px 0px 40px -2px rgba(0,0,0,0.63);
border-radius: 5px;
font-family: 'Merriweather', serif;
font-size: 20px;
}
.container {
height: 420px;
background-color: blue;
top: 30%;
width:100%;
background-image: linear-gradient(to right top, #a62a71, #963680, #83408c, #6d4993, #565096, #4560a6, #2e70b2, #007fba, #009ed0, #00bee0, #1cddeb, #5ffbf1);
box-shadow: 0px 0px 40px -2px rgba(0,0,0,0.63);
transform: translateY(50%);
}
.internal-width {
padding: 0;
width: 85vmin;
margin-left: auto;
margin-right: auto;
display:flex;
flex-wrap: wrap;
justify-content:space-between;
}
.box{
flex-basis:calc(94% / 3);
height: 100px;
margin:1%!important;
background-color: #22584F;
transition: all 0.2s ease-in-out;
border-radius: 20px;
margin-bottom: 50px;
margin-top: 50px;
transition: all 0.2s ease-in-out;
}
.box:hover
{
box-shadow: 0px 0px 20px 5px rgba(0,0,0,0.4);
cursor: pointer;
transform: scale(1.1);
}
@media screen and (max-width: 1000px) {
.box{
float: none;
}
}
<div class="undefinedheader">
tu może być tyuł
</div>
<div class="container">
<div class="internal-width">
<div class="box"><a href=""></a></div>
<div class="box"><a href=""></a></div>
<div class="box"><a href=""></a></div>
<div class="box"><a href=""></a></div>
<div class="box"><a href=""></a></div>
<div class="box"><a href=""></a></div>
</div>
</div>