If Kendra forgot the gift card for three months, the card loses $5 each month.
To calculate the total amount lost after three months:
\[ \text{Total lost} = \text{Amount lost per month} \times \text{Number of months} \]
\[ \text{Total lost} = 5 \times 3 = 15 \]
So, Kendra lost $15. The answer is 15.