Nav Bar not responsive

Everything isn't working as fine as i thought :) the navbar isnt scaling with the different viewports. Solution would be to wrap individual blocks around divs and give widths to equal 100%. Make the last button block text-align center
.nav_bar{
max-width: $default_width;
height: 100px;
border: 2px solid orangered;
}
.first_box{
width: 24.9%;
height: 100px;
border: 2px solid red;
float: left;
text-align: center;
}
.second_box{
width: 37.4%;
height: 100px;
border: 2px solid palevioletred;
float: left;
}

.third_box{
    width: 37.4%;
    height: 100px;
    border: 2px solid darkolivegreen;
    float: right;
    text-align: center;
}