<<If, for whatever reason, you DO want to specify the other lower basis ($18) shares, you might be able to do so (but, as KAT points out, I'm not sure why you would want to do so).>>

Might want to generate a short-term loss carryover (the higher basis shares were held long-term). Wouldn't matter for this year because no other gains or losses, but for next year could be better. But the original question indicated only an interest in generating a $3,000 loss for the current year.

KAT in Chicagoland