Help me decide if it is worth it to complete a promotional balance transfer of 2.99% APR for 6 months?

The others are right, it's not worth it. Also check the terms of the balance transfer to see if back-interest would accrue if the balance transfer amount is not paid off in 6 months. If you can pay an extra $215 this month and $150 for the next six months, you could have Discover paid off in 3 months, or ML paid off in 4.