Chris O'Donnell may have built an entire career upon playing suitors ("Scent of a Woman") and sidekicks ("Batman & Robin"), but on ABC's "The Practice," he's playing a man accused of murdering his pregnant wife.

That's quite a leap for the 30-year-old, who just became a dad for the third time with wife Caroline Fentress, 30. Their son Charlie was born on July 11, joining sister Lily, 3, and brother Chip, 2.

Speaking to PEOPLE about the recent arrival, O'Donnell said, "Lily was disappointed he wasn't a girl ... but they are real proud of their little brother."

And though Lily and Chip "were acting up," the first week Charlie was home, the proud papa says, "they better get used to it because I'd like to have more."

Of his three-part TV role as Brad Stanfield on "The Practice" (which kicked off last Friday), O'Donnell said: "It's exciting, because it's such a different pace than what I'm used to."

He likens the new experience to "when I did a show on Broadway last year, which was an Arthur Miller play ('The Man Who Had All the Luck'). I mean, it's such a great role, it doesn't matter what situation you're in or where you are doing it."

The downside of his career is that acting sometimes takes him away from his family. (He is currently shooting the movie "Kinsey," with Liam Neeson and Laura Linney, in New York City). "It is tough," he says. "I mean, fortunately they are close by. They are up in Maine, and we have what I consider a normal schedule. Granted, they are very long days, but they are only five days a week."

But that hasn't always been the case. Movies he's done in the past have sometimes required "six days a week," he says, "and your day off was Tuesday, and you really didn't feel like you were part of society! But I am able to get off on the weekends and see them."