There is a detailed post about creating 3D Clips For Prescription Glasses that addresses this precise issue.

The simplified version is: use a set of clip-on sunglasses, harvest the lenses from a set of theater glasses, pop the lenses out of the clip-on, use them as a template to cut replacements from the 3D glasses, and pop in your new 3D lenses.

Note: I'm comfortable wearing 3D glasses over my glasses so I have not used this technique.

There are a couple ways to go about a clip-on solution that might be appealing to people not comfortable wearing contacts or glasses over glasses.

DIY Version

There is a detailed post about creating 3D Clips For Prescription Glasses that addresses this precise issue.

The simplified version is: use a set of clip-on sunglasses, harvest the lenses from a set of theater glasses, pop the lenses out of the clip-on, use them as a template to cut replacements from the 3D glasses, and pop in your new 3D lenses.

Note: I'm comfortable wearing 3D glasses over my glasses so I have not used this DIY technique.

Commercial options

Companies have figured out that people who wear glasses sometimes want to watch 3D movies and have started producing clip-on solutions. The notable boon to a commercial option is not having to deal with cutting, trimming, fitting, and aligning polarized lenses into a frame they weren't designed for.

A couple examples include:

• Circular Polarized Clip on 3D Glasses for RealD
• Passive Linear Polarized Clip On 3D Glasses

A quick search on Amazon yields a lot of options.

Caveats

1. Different 3D technologies use different methods of polarization and it's important to have versions for each
2. This only works for passive 3D technologies. Active 3D is a whole different ball of wax.