Row size overhead
Those 3 extra are from the NULL Bitmap. According to Paul's post, it's on every row save for those that are all SPARSE across the columns (beginning in SQL Server 2008).
And according to a line in this BOL post, the NULL Bitmap is = 2 + ((number_columns_in_clustered_index + 7) / 8). In your case, 3.
I partially agree with @Matt, the 2 bytes are required for NULL bitmap which is correct.
However, the last byte is consumed by the number of columns per bit. Meaning, If I have 6 columns in my table then I'll require 1 byte (6-bits), or If I have 12 columns then I'll require 2 bytes (12-bits).
In your case there are 3 columns hence it has taken just 1 byte.