SOURCE

<div class="box">
    <div class="a1"></div>
    <div class="a2"></div>
    <div class="a3"></div>
</div>

.box{
    width: 140px;
    height: 40px;
    background: #000;
    border-radius: 40px;
    display: flex;
    justify-content: space-evenly;
    align-items: center;
}
.box >div{
    border-radius: 50%;
    background: #fff;
    width: 34px;
    height: 34px;
    
}
.box .a1{
    background: red;
}
.box .a2{
    background: yellow;
}
.box .a3{
    background: green;
}