Stolzenberg: <3. ... a6> has some advantages compared to 3. ... Nf6:1.: It excludes the Queen's Gambit Accepted (D23) 4. Qa4+, the Mannheim variation.
2.: After the most popular answer 4. e3 Black can choose between
a) 4. ... Nf6 transposing to the Queen's Gambit Accepted, Classical (D27)
b) 4. ... Bg4, the Alekhine defence and
c) 4. ... b5, the Haberditz variation.
3.: Black does not need to be afraid of 4. e4. It could follow for example:
4. ... b5 5. a4 Bb7 (attacking e4) 6. axb5 axb5 7. Rxa8 Bxa8 8. Nc3
Now 8. ... b4 would find the answer 9. Qa4+, but Black has two other possibilities:
a) He can give the pawn back with 8. ... e6 9. Nxb5 Bxe4 10. Bxc4 and now he can choose between 10. ... Nf6, 10. ... Bb4+ or 10. ... Bxf3.
b) He can defend the pawn with 8. ... c6, for example 9. Be2 e6 10. 0-0 Nf6 11. Bg5 Be7.
I wonder why <3. ... a6> is not more popular.