Target iPad3 but not iPad4

Question 1

You can't get any difference by using media queries or user-agents:

Media queries are the same
User Agent contains only iPad word, not version. Other info in UA is about iOS or Safari browser. iPad3 has iOS6 be default, but in can be updated for new versions. Safari depends from iOS version, so system and browser information is imprecise.

So there is only javascript+iOS feature detecting like was with iPad and iPad2 difference in event.acceleration, but there is no different in features of iPad3 and iPad4.

So you can't detect iPad3 only. May be you post here the problem with iPad3 and we can help you to solve it?

BUT:

If you use WebView component, you can do it in it. You can detect in by systemInfo.machine string in Objective-C:

NSString *machineName = [NSString stringWithCString:systemInfo.machine encoding:NSUTF8StringEncoding];

machineName for iPad3 are iPad3,1, iPad3,2 and iPad3,3. iPad4 are iPad3,4, iPad3,5 and iPad3,6 (don't ask why:)

And in WebView you can change your css file to iPad3.css by using WebResourceLoadDelegate for example.

Question 2

Confirming that you can't get any difference by media queries or user-agents, I've found a difference between iPad3 and iPad4 using WebGL and js.

Using EXT_texture_filter_anisotropic and its extension object MAX_TEXTURE_MAX_ANISOTROPY_EXT you can return the maximum available anisotropy for a texture.

IPad3 returns 2, iPad4 returns 16

I guess this is due to their different GPU.

Demo

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Anisotropy</title>
</head>

<body>

    <p style="font-size: 100px;" id="anisotropy_val"></p>   

     <canvas id="webGLCanvas" width="400" height="400">
     </canvas>

</body>
<script>

function check_max_anisotropy() {

var canvas = document.getElementById("webGLCanvas");
var gl = canvas.getContext("experimental-webgl");


var ext = (
  gl.getExtension('EXT_texture_filter_anisotropic') ||
  gl.getExtension('MOZ_EXT_texture_filter_anisotropic') ||
  gl.getExtension('WEBKIT_EXT_texture_filter_anisotropic')
);

if (ext){
  var max = gl.getParameter(ext.MAX_TEXTURE_MAX_ANISOTROPY_EXT);
}

document.getElementById("anisotropy_val").innerHTML = max;

}

check_max_anisotropy();

</script>
</html>