There are now 3 copies of him in Windhelm. If I kill all of them, will he be dead forever? Or does he keep spawning infinitely now?

The "original" Letrush is the one you find in Riften at the Bee and Barb inn. It should be safe to kill/disable all his clones.

I play on 360, so I can't disable them. Are none of the ones in Windhelm the orginal? And, if I find the original and kill him, will that stop the ... Attack of the Clones?

Sorry, I'm a Star Wars nerd. Forgive the horrid pun.

I've got 10 now and I kinda like them. He's marked down as "essential" for me so none of them can be killed (I've tried dragons, giants, mudcrabs, giant mudcrabs) but they still turn up. Only option if you want to get rid of them is use the console if on PC.

Edit: No on 360 there's not much you can do about it. If you've done the quest for him already then he's already gone forever. It's usually after said quest the "Clones" start to show up.

I was on my way to Winterhold when I hear some orc yell 'Maven Black-Briar sends her regards!'. I turn to see a mercenary slaying Letrush way out in the frozen wastes. is that....normal?

Yes, he stole her horse after all.