So far as we know, you are right: each summons is an independent event, with a 3% (or 2%, or 1.5%, or whatever it is) chance of summoning a 5* hero. Repeated summons give you repeated chances, but your chances on your tenth summons are not improved by having failed to draw a 5* with your previous nine summons. They remain 3% (or 2%, or 1.5%, or whatever it is).
