A simple help to topic https://www.facebook.com/groups/javascriptbrasil/permalink/511054952319536/

verificar.js

function verificar() {
  var dados = document.getElementById("dados"),
      t1 = document.getElementById("t1").value,
      t2 = document.getElementById("t2").value,
      t3 = document.getElementById("t3").value;

    if (t1 == t2 && t2 == t3 && t1 == t3) {
      dados.triangulo.value = "Isosceles";
    }
    
    else if (t1 == t2 || t2 == t3) {
      dados.triangulo.value = "Equilatero";
    }
    
    else {
      dados.triangulo.value = "escaleno";
    }
}