.uc-mission {
  position: relative;
  /* ... твой фон и всё прочее ... */
}

/* Первый лист */
.uc-mission::before {
  content: '';
  position: absolute;
  left: 10vw;
  top: 80vh;
  width: 48px;
  height: 48px;
  background: url('https://static.tildacdn.com/tild6638-3961-4566-a437-393061646661/leaf.png') no-repeat center/contain;
  opacity: 0.7;
  pointer-events: none;
  z-index: 20;
  animation: flyLeaf1 12s linear infinite;
}

/* Второй лист */
.uc-mission::after {
  content: '';
  position: absolute;
  left: 70vw;
  top: 90vh;
  width: 40px;
  height: 40px;
  background: url('https://static.tildacdn.com/tild6638-3961-4566-a437-393061646661/leaf.png') no-repeat center/contain;
  opacity: 0.6;
  pointer-events: none;
  z-index: 20;
  animation: flyLeaf2 14s linear infinite;
  animation-delay: 3s;
}

/* Третий лист — через вложенный псевдоэлемент */
.uc-mission .leaf-third {
  position: absolute;
  left: 40vw;
  top: 95vh;
  width: 36px;
  height: 36px;
  pointer-events: none;
  z-index: 20;
}
.uc-mission .leaf-third::before {
  content: '';
  display: block;
  width: 36px;
  height: 36px;
  background: url('https://static.tildacdn.com/tild6638-3961-4566-a437-393061646661/leaf.png') no-repeat center/contain;
  opacity: 0.6;
  animation: flyLeaf3 17s linear infinite;
  animation-delay: 6s;
}

/* Анимации */
@keyframes flyLeaf1 {
  0% { left: 10vw; top: 80vh; transform: rotate(-20deg) scale(0.8);}
  40% { left: 25vw; top: 55vh; transform: rotate(8deg) scale(1);}
  60% { left: 38vw; top: 48vh; transform: rotate(32deg) scale(1.1);}
  100% { left: 70vw; top: 8vh; transform: rotate(70deg) scale(0.7); opacity: 0.1;}
}
@keyframes flyLeaf2 {
  0% { left: 70vw; top: 90vh; transform: rotate(35deg) scale(1.1);}
  50% { left: 60vw; top: 60vh; transform: rotate(-14deg) scale(0.95);}
  100% { left: 20vw; top: 8vh; transform: rotate(-80deg) scale(0.6); opacity: 0.15;}
}
@keyframes flyLeaf3 {
  0% { margin-left: 0vw; margin-top: 0vh; transform: rotate(90deg) scale(0.7);}
  50% { margin-left: 15vw; margin-top: -28vh; transform: rotate(60deg) scale(1);}
  100% { margin-left: 55vw; margin-top: -90vh; transform: rotate(-35deg) scale(0.5); opacity: 0.1;}
}