Han rolls an attack, hit-hit-blank

Defender makes him re-roll the two hits (R7 Astromech) and leaves Han with blank-blank-focus.

Does Han's ability (old-Han, young-Han, Wave 2 Han...whatever Han was here in 2014) allow him to re-roll all three dice, or just the one blank (per rules saying you can only re-roll once)?

Han will be able to reroll only the dice not already rerolled.

Han would only be able to re-roll the one blank.

When attacking, you may reroll all of your dice. If you choose to do so, you must reroll as many of your dice as possible.

HanShoot can still only reroll a given die once. If R7 makes him reroll dice before he gets the chance to modify them then he can't roll them later. Han also couldn't reroll a die turned by a Sensor Jammer.