STRATEGY: Upon reading any GRE Multiple Choice question, we should always ask, Can I use the answer choices to my advantage? (more on this here).
In this case, we can easily test the answer choices.
From here, I'd typically give myself up to 20 seconds to identify a faster approach, but I can already see that we can eliminate three of the five answer choices, which means we need only test 1 answer choice.

Here's how we can eliminate 3 answer choices....

Given: In 3 years, Sam will be 3 times as old as Chris
So, 3 years from now, Sam's age must be a multiple of 3.
Check the answer choices...

A. If Sam's present age is 16, then in 3 years he'll be 19 years. Since 19 is not a multiple of 3, we can eliminate A
B. If Sam's present age is 18, then in 3 years he'll be 21 years. Since 21 IS a multiple of 3, we'll keep B for now.
C. If Sam's present age is 20, then in 3 years he'll be 23 years. Since 23 is not a multiple of 3, we can eliminate C
D. If Sam's present age is 22, then in 3 years he'll be 25 years. Since 25 is not a multiple of 3, we can eliminate D
E. If Sam's present age is 24, then in 3 years he'll be 27 years. Since 27 IS a multiple of 3, we'll keep E for now.

We're left with B and E, which means we need only test 1 answer choice. For example, if we test B, and it works then we're done. If it doesn't work, then the correct answer must be E.

Let's test B) 18
If Sam is presently 18 years old, then Chris must be 4 years old (since Chris is 14 years younger than Sam)
So, in three years Sam will be 21 and Chris will be 7, which means Sam will be three times as old as Chris.
Perfect!

Answer: B