.container {
  display: flex;
  gap: 20px;
  flex-wrap: wrap;
}

.box {
  width: 300px;
  height: 300px;
  border: 1px solid #000;
}

#zadatakA {
  background: linear-gradient(to top right, yellow 0%, red 50%, black 100%);
}

#zadatakB {
  background: radial-gradient(circle at right, white, darkblue);
}

#zadatakC {
  background: linear-gradient(to bottom, 
    red 33.3%, 
    white 33.3%, white 66.6%, 
    blue 66.6%);
}