What's a main difference between CRS1, CRS2 and CRS3

Even though officially they're still available, essentially CRS1xx and CRS2xx are obsolete.

So technically, main difference is load that can they handle?

All CRS switches can switch wirespeed. Some can do wirespeed slightly more complicated stuff. With ROSv7 beta some (but not all) CRS3xx devices can even route wirespeed.

But the big difference is that CRS3xx are contemporary products offering e.g. 10Gbps interfaces. CRS1xx and CRS2xx are not.