It follows a report in the New York Post, quoting Potter producer David Heyman as saying that the trio could outgrow their roles.
Alfonso said Dan Radcliffe, Emma Watson and Rupert Grint could continue playing Harry, Ron and Hermione.
"I just hope they keep the same cast for the whole thing," he added.
The trio, who sign for one film at a time, have already started work on the fourth movie, Goblet Of Fire.
In the film, they are supposed to be 14, which matches Emma's age. Dan will turn 15 this summer, and Rupert will turn 16.
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"I don't think Dan is going to get way much taller or suddenly grow another eye. Same with Emma, and Rupert is okay," said Alfonso.
"In the last one they're supposed to be 17, Say they're 19, I think they're going to be very good. That would be very special."
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