The simplest answer to this is No. The “rule” that an engagement ring should be 3x (some say 2x) your monthly salary may have been perpetuated by cunning jewelry retailers who want men to spend more at their shops. There are no hard and fast rules about how much you should spend on an engagement ring. In saying this, however, it doesn’t mean you can be scrimpy and stingy when buying an engagement ring (choosing the smallest stone, or the cheapest ring available, even if it looks tacky as hell).
When you’re ready to pop the question, find out (subtly) what kind of ring your girlfriend wants. Look at her other pieces of jewelry and make sure it will “blend” with these. Also take into consideration her personality and lifestyle. If she’s into mountain climbing and shearing sheep, a big rock on her finger will just impede her activities. Then, check your budget. If you can afford to spend 2x (or even more) of your salary on an engagement ring, by all means, go for it. If you can’t then get one that is more affordable.