The total amount of cookies he can eat in a day is 13 x 3 = <<13*3=39>>39.
The total amount of cookies he can eat in a month is 39 x 20 = <<39*20=780>>780.
Therefore, the total amount of cookies he can eat in 3 months is 780 x 3 = <<780*3=2340>>2340.
#### 2340