Definitely it is time to change them, Some do it every year (they are not expensive). Personally I consider it unnecesary, but with age they start loosing their performance (electrode erosion) Cracked ceramic is particularly dangerous, because broken piece can seriously damage cylinder and piston. At least 3 years ? way too much for my liking.
Which one to get? Just find on the net what is for your engine, some of then can be bought at CTC, others definitely from dealer. Do not go for "the best", you are not racing on the water aren't you ?