Samsung S3 i9300 rooted with Heimdall.  Am I able to unroot with ODIN?
You don't "unroot" per se with Odin; you just flash a stock, unrooted ROM that wipes out everything currently on the phone.  It shouldn't matter whether you use Odin or Heimdall.

Note also that unrooting won't matter if you've flashed anything with Odin, since that also voids your warranty and is recorded on the device.