h3.primary {
  position: relative;
  display: block;
  width: fit-content;   /* テキスト幅だけにする */
  margin: 0 auto;       /* ブロック要素なので中央寄せが効く */
  padding: 0 1em;
  text-align: center;
  color: #3399cc;
  font-size: 26px;
  line-height: 1.4;
}

/* 疑似要素は同じ */
h3.primary::before,
h3.primary::after {
  content: "✦";
  position: absolute;
  top: 50%;
  transform: translateY(-50%);
  font-size: 0.9em;
  color: #3399cc;
}

h3.primary::before { left: 0; }
h3.primary::after  { right: 0; }

.primary-sub {
  display: block;
  font-size: 24px;
  text-align: center;
  margin-top: 0.8em;
  color: #3399cc;
  margin-bottom: 1em;
  line-height: 1.4;
  font-weight: bold;
}
/* SP用 */
@media screen and (max-width: 767px){
  h3.primary {
    font-size: 18px;
  }
  .primary-sub {
    font-size: 16px;
  }
}
@media screen and (min-width: 768px){
  br.sp {
    display: none;
  }
}