We raise Simmental, and breed all of our heifers at 14 months. We have had no problem with longevity so far.
I agree with what others have posted here, since you are so new I would ask the guy you bought them from what bull he would use. If all three are heifer safe, then you need to know what direction you are taking with your cattle. I personally love the 3/4 blood heifers. I can take those, and cross them with another half blood and still get some really nice cattle that can be registered, however, I can also breed them to a purebred Simmental bull and get purebreds. But, you need to look at the phenotypes of the heifers and bulls and see which will match up the best. Also, look long and hard at the EPD's. The Simmental breed has really nailed those pretty close, and they will tell you which direction you might want to go. The ASA data base has a great tool, where you can plug in the heifers registration numbers, and the bulls registration numbers, and get an estimated EPD on each mating! So you can look at what the calves might look like on an EPD basis.
If you need help, feel free to private message me. I can help you look up information and give you some ideas on directions depending on what your end goal is.
And of course, WELCOME to the Simmental breed. I love my Simmental cows...