For his third Spy Kids movie, due July 25, director Robert Rodriguez is adding some depth. No, Carmen (Alexa Vega) and Juni (Daryl Sabara) aren't getting a drug-addiction subplot. ''The story line takes the kids into a virtual-reality videogame,'' says Rodriguez. ''Once [the characters] go into that world, the audience puts on the glasses, and it's all 3-D.'' Rodriguez promises that the technology has advanced significantly since the dark days of 1983's Jaws 3-D, thanks to high-definition digital filmmaking. But can it make 82-year-old Ricardo Montalban, who plays the kids' wheelchair-bound grandpa, a spry spy? ''He looks like Khan again,'' swears the director. ''He can jump and shoot and surf lava. He's so pumped.''