Is it bad business etiquette to show vendors each others' quotes?

Suppose I am looking to purchase parts for my company and I quote three vendors. Suppose Company A is lowest. Would it look bad to send copies of Company A's quote to B and C, and see if they could go lower? How about then go back to Company A and try to knock them down some more? Is there any way I could get in trouble for doing this?

If you give their bid to a competitor you will probably lose them from your source chain in the future. You can indicate that you have lower bids and give all of them a chance to resubmit.