I believe with the right research that most species within reason can be kept as a first snake. I've kept a lot of different species including the three species you've mentioned. My personal favs are garters, although I recommend corns too. I love garters because they are so active during the day and make a great display pet. They come in a fantastic array of naturally occuring forms. They are voracious feeders. Mine feed on a mixture of bits of trout and pinkies. They are a lot faster than corns to get hold of but are fun to handle and I've never had any problems. I love them and they are a pleasure to keep.
Corns are great too - generally placid, easy to handle and great feeders. You can get them in a vast array of morphs too. From personal experience they aren't anywhere as active as garters during the day. They grow a little larger but are slower to handle.
I've kept royals and I can understand why people love them, but for me they just didn't do it. I like active animals that I can watch going about their business. But as I said each to their own.
Summing up I think go with which ever species you like the look of and best suits what you want from a pet snake. For me it was a smallish species that was easy to feed, good to hande and fun to watch during the day. That's why garters got my vote 15 years ago and I still keep a variety of them now. However, your criteria may be slightly different.