You have a reportable sale here. On June 28, 2017 CST Brands was acquired by Alimentation Couche-Tard.
"Under the terms of the merger agreement, CST stockholders will receive $48.53 in cash per share, without interest, as a result of the closing of the transaction."
So you have the choice of either amending or doing nothing and seeing if the IRS catches it some day and contacts you about the missing 1099-B, and maybe they won't.
If you only had 3 shares we're not talking about a lot of money here and depending on your tax bracket and assuming both a capital gain and a long term holding, the gain could be taxed at 0%. Even if your capital gains tax rate isn't 0% for most people it's 15% which isn't bad. And of course you could have a loss, which could result in a small refund.
You might try amending you income tax return, entering the information needed to report the sale, and if you taxes don't change then you really don't have to amend. If you chose not to amend then don't save that tax file because you don't want to overwrite your "as filed" income tax return.
FAQ on amending:
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
Tom Young
