His age 15 years from now will be x+15.
Three times his age 5 years ago is 3(x-5).
If both are equal, we can write:

The equation representing William's claim is x+15 = 3(x-5).
We can calculate its present age as:

His actual age is 15 years.
Answer:
The equation representing William's claim is x+15 = 3(x-5).
His actual age is 15 years.