The three adjectives in the sentence are; numerous, beneficial, and some.
What is an adjective?
An adjective is a word that is meant to qualify something or give it a deeper meaning. In the sentence provided, the word numerous qualifies the word studies by letting us know that there is more than one study.
Also, the word beneficial describes the nature of the exercise. So, we see the usefulness of exercise. The word tells us that exercise is very important.
Lastly, the word some is an adjective that describes the kinds of cancer that are obtainable. So, these three are the adjectives.
Complete Question;
Select the three adjectives. Don't select any articles (a, an, or the).
Numerous studies have found that exercise is beneficial because it reduces the risk of illnesses such as stroke, diabetes, and some forms of cancer.