Electronic Blackjack - hit after doubling down.

[tableplay]

You have a 10 vs dealer 8. You double down and draw a 3. Your double down is now likely a losing hand. But hitting again, you have a chance to improve your hand. You might still bust or draw a 2 or 3 and still lose, but you have the chance to improve your hand and that makes everything +EV.

No. It absolutely does not make it +EV for this 10 versus 8 spot. It just reduces the degree to which it is -EV (i.e. you would lose less on average in this 10 vs. 8 spot than if you didn't have the option to take a fourth card).