Hi Brooke. When you get into more advanced science experiments, figuring out an equation from a series of measurements is a major part of your work!
Fortunately, in your question, it is not too hard to see the relationship between the numbers:
Start with 3.
3 times 5 is 15.
15 times 5 is 75.
75 times 5 is 375.
Do you see the pattern? You can write the general equation using the number of weeks as an exponent. Then you can figure out the number of bird infections in 52 weeks (or years, if you wish!)
Hope this helps,
Stephen La Rocque.