.conteneur{
  border: 1px solid black;
  box-sizing: border-box;
  background-color: #0FA;
}
.p1{
  background-color: #EE0;
}
.p2{
  float: left;
  background-color: #0AF;
}
.p3{
  background-color: #C08;
}
.p1::before{
  content: "Texte inséré 
  avant le paragraphe 1 ! ";
}
.conteneur::after{
  display: table;
  content: "";
  clear: both;
}