When, and if, you get confirmation from the landlord make sure you get it in writing
I've got all three. Corns are typically bombproof as a species. I've had mine 4 years, he has never hissed or bitten, never missed a meal and has always been just perfect.
Garters are good as well, however I've found them a little nervy as youngsters and mine regularly musks all over me. But he favours my mum over me, he's never liked me
Royals are brilliant, I love all of mine, and they seem to go against normal royal behaviour, always bounding around, very active and inquisitive. But the feeding routines
I've nearly throttled half of mine because they just decide they don't want to eat for 3 months or something stupid like that. For a new owner that can be concerning, my first royal was my second snake, and after having a corn thats a dustbin, I nearly went insane when my royal had her first fast!
They're all great species, and if you can accept that they wont eat for no reason then royals are fine too. Do your research and you'll be fine with any of those species