Since Mary first weighed 99 pounds, after she dropped a dozen pounds she weighed 99-12=<<99-12=87>>87 pounds.
Twice the weight she initially lost is 12*2=<<12*2=24>>24 pounds.
So, when she added back twice the weight that she initially lost, her weight jumped to 87+24=<<87+24=111>>111 pounds.
Three times more than she initially had lost is 12*3=<<12*3=36>>36 pounds.
Therefore, when Mary lost 3 times more than she initially lost, her weight dropped to 111-36=<<111-36=75>>75 pounds.
Finally, when Mary gained back half of a dozen pounds, her final weight was 75+6=<<75+6=81>>81 pounds.
#### 81