"On the third floor stood a salesman with [number] artificial legs"

Solution 1:

Well, your problem is that it's ambiguous, mostly the first sentence. When I first read the sentences, I thought he was wearing two artificial legs. Then I read the next two sentences and I thought "Wait hold on!" People only have two legs. I reread it slowly and realized you meant carrying two, three, and four, legs, respectively. Then I was plain confused what you meant.

To remove this ambiguity, we must rephrase.

On the first floor stood a salesman carrying two artificial legs.

On the first floor stood a salesman wearing two artificial legs.

The meaning is now clear. No more creepy three-legged salesmen.