How to use a single array to implement three stacks in JavaScript

function TripleStack() {
  this._array = [];
  this._lengths = [0, 0, 0];
}
TripleStack.prototype._getLength = function(stack) {
  return this._lengths[stack-1];
};
TripleStack.prototype.push = function(stack, val){
  const idx = this._getLength(stack) * 3 + stack - 1;
  this._array[idx] = val;
  this._lengths[stack - 1]++; // we reference stacks w/ 1, 2, 3 but internally stack lengths start their indices at 0
};
TripleStack.prototype.pop = function(stack) {
  let len = this._getLength(stack);
  let val;
  if (len) {
    const idx = (len - 1) * 3 + stack - 1;
    val = this._array[idx];
    delete this._array[idx];
    this._lengths[stack - 1]--;
  }
  return val;
};
TripleStack.prototype.peek = function(stack) {
  let len = this._getLength(stack);
  let val;
  if (len) {
    const idx = (len - 1)  * 3 + stack - 1;
    val = this._array[idx];
  }
  return val;
};
TripleStack.prototype.isEmpty = function(stack) {
  return this._getLength(stack) === 0;
};