3 State CSS Toggle Switch

.switch-toggle {
   float: left;
   background: #242729;
}
.switch-toggle input {
  position: absolute;
  opacity: 0;
}
.switch-toggle input + label {
  padding: 7px;
  float:left;
  color: #fff;
  cursor: pointer;
}
.switch-toggle input:checked + label {
  background: green;
}
 <div class="switch-toggle switch-3 switch-candy">

  <input id="on" name="state-d" type="radio" checked="" />
  <label for="on" onclick="">ON</label>

  <input id="na" name="state-d" type="radio" checked="checked" />
  <label for="na" class="disabled" onclick="">N/A</label>

  <input id="off" name="state-d" type="radio" />
  <label for="off" onclick="">OFF</label>

</div>

Try something like this:

.switch-toggle {
  width: 10em;
}

.switch-toggle label:not(.disabled) {
  cursor: pointer;
}
<link href="https://cdn.jsdelivr.net/css-toggle-switch/latest/toggle-switch.css" rel="stylesheet" />

<div class="switch-toggle switch-3 switch-candy">
  <input id="on" name="state-d" type="radio" checked="" />
  <label for="on" onclick="">ON</label>

  <input id="na" name="state-d" type="radio" disabled checked="checked" />
  <label for="na" class="disabled" onclick="">&nbsp;</label>

  <input id="off" name="state-d" type="radio" />
  <label for="off" onclick="">OFF</label>

  <a></a>
</div>

This will start with N/A as the default option (via checked="checked"), but make it unselectable later (by using disabled)

JSFiddle Demo (Simplified)