Indeed, the line is relatively rare, but the move 3....a6 seems to become more and more popular the last couple of years. Even Magnus Carlsen has played it twice in blitz recently, against Aronian and Grischuk.
The move 4.cxd5 is the most popular move and arguably the only way to fight for an advantage. After 4.Nf3, black has the choice to transpose to a favorable version of the QGA by 4....dxc4. As the knight is already on c3, black avoided the most critical lines of the QGA.
After 4.cxd5 exd5, in Mamedyarov-Vallejo Pons, white went for the aggressive 5.Nf3 c6 6.e4!? dxe4 7.Ng5 Be7 8.Bc4. Now, 8....Bxg5 would be a mistake as white regains the piece by 9.Qh5, with a strong initiative. Instead, black played 8....Nh6, defended correctly after 9.Ngxe5 Nf5 and made a draw.
Another try to spice up the game is 5.Qb3!?, which was played recently by the Russian top GMs Nepomniachtchi, Matlakov, Vitiugov and Svidler. After the logical 5....c6, white obtains a dangerous initiative after 6.e4 dxe4 7.Bc4 Qe7 8.a4! (to prevent ...b5), which was played by Svidler. In the three other games, after 5.Qb3, black continued with 5....Nc6. After 6.Nf3 Bb4 7.Bf4, white is probably slightly better, as the black knight on c6 is somewhat misplaced.
1.d4 d5 2.c4 e6 3.Nc3 a6 4.cxd5 (4.Nf3 dxc4) exd5 5.Qb3 (5.Nf3 c6 6.e4 dxe4 7.Ng5 Be7 8.Bc4 Nh6 (8...Bxg5 9.Qh5) 9.Ngxe4 Nf5) 5...Nc6 (5...c6 6.e4 dxe4 7.Bc4 Qe7 8.a4) 6.Nf3 Bb4 7.Bf4