Club soda is plain water-system water with carbon dioxide and sodium (salt) added.
Seltzer is plain water-system water with carbon dioxide added.
"Sparkling water" may be naturally carbonated, with or without naturally occuring minerals; it also just be plain water with carbon dioxide added. The main difference between sparkling water and the other two is the price people are willing to pay.
Whether or not they can all be used for "scotch and soda" depends on the person who makes the standards for the bar.
We had a thread on this some time ago; try doing a search.