Javascript slider to change variable dynamically

The simplest answer to the question is that you can set your variables from the slider's handlers

 slide: function( event, ui ) {
    $('amount1').val( ui.value );
    // This is the value that you just dragged the slider to, there it is,
    // in a variable
    x = ui.value;
  }

Without knowing what you needed I whipped up an example that should show you a working example where I extract the values from the sliders and apply it to the color of the circle in the canvas. This code works with the HTML you posted initially

function drawOnCanvas () {
   var rgb =[
       $('#slider-1').slider("value"),
       $('#slider-2').slider("value"),
       $('#slider-3').slider("value")
   ];
   var c = document.getElementById("myCanvas2");
   var ctx = c.getContext("2d");
   var color = "rgb("+ rgb.join(',') + ")" ;
   ctx.fillStyle = color;
   ctx.beginPath();
   ctx.arc(100, 100, 100, 0, Math.PI*2, true);
   ctx.closePath();
   ctx.fill();
}

function createSlider(slider, boundTextField) {
   slider.slider({
      orientation: "vertical",
      range: "min",
      min: 0,
      max: 255,
      value: 0,
      slide: function( event, ui ) {
        boundTextField.val( ui.value );
        drawOnCanvas();
      }
  })
}

$(function() {
    createSlider($( "#slider-1" ), $( "#amount1" ));
    createSlider($( "#slider-2" ), $( "#amount2" ));
    createSlider($( "#slider-3" ), $( "#amount3" ));    
});