<div id="star"></div>

#star {
    width: 0;
    height: 0;
    border-top: 58px solid #f00;
    border-left: 80px solid transparent;
    border-right: 80px solid transparent;
    position: relative;
    margin: 100px;
}

#star::before {
    content: "";
    display: block;
    width: 0;
    height: 0;
    border-top: 58px solid #0f0;
    border-left: 80px solid transparent;
    border-right: 80px solid transparent;
    position: absolute;
    top: -60px;
    left: -77px;
    transform: rotate(-72deg);
}

#star::after {
    content: "";
    display: block;
    width: 0;
    height: 0;
    border-top: 58px solid #00f;
    border-left: 80px solid transparent;
    border-right: 80px solid transparent;
    position: absolute;
    top: -60px;
    left: -83px;
    transform: rotate(72deg);
}