Originally Posted by
invisible
...gas at foreign gas stations and get awarded with 3x points?
Category bonuses are driven by merchant classifications, not by what you purchase. Classifications are established by the company that processes the merchant's card transactions, not by the card issuer (Citi).
In the USA many gasoline stations are also convenience stores, and there is not any easy way to know how their merchant account is configured until you make a purchase. If you have used any MasterCard to buy gasoline at some of the stations you have in mind it may be possible to see how the merchant is classified. .