If shares were withheld, they were not sold. Your broker won't indicate a transaction on the withheld shares. What happens to them is they go back into the company's pot of stock that they can then use for other reasons, like loan guarantees or employee/management incentives. The company took the value of that withheld stock at vesting time (usually end-of day, you can validate this if you look at your pay stub and note the difference between that one and the previous pay period) and dumped that into your tax bucket which gets reported on your W2.
The company added the total value of the RSU to your income. Again, you can validate that by looking at your pay stub covering the vesting date, and comparing it to your previous pay stub. So your tax is calculated by TT automatically. Since this is income, there is no capital gain or loss until you sell the remaining share. So you don't enter it at all in TT. When you sell that third share, the cost basis at that time to used to calculate the gain or loss on that share is the value at vesting date. Because you will already have paid the tax on the full value of the RSU when you file your taxes.
If you haven't figured it out yet, you should save your two pay stubs, the grant and vesting documents, and you can call your broker to get a breakdown of what buckets that the withheld shares were put in, like FICA bucket, Medicare bucket, State tax bucket, etc.
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