I would definitely take Karen up on her offer, a three page resume is too long. The only time a resume of that length is necessary is for academic positions that require you to list all your publications.

As far as listing all your volunteer work - take 3 or 4 that emphasize the skills they are looking for and list your responsibilities. For example:

Volunteer - Buffalo Animal Shelter (date - date)
Supervisor - Mary Mary
*Responsible for researching and coordinating Fundraising Activities
*Aided adoption coordinator i screening and approving adoption applications
*Responsible for creating, organizing and distributing literature to potential adoptive families

Also - I would strongly suggest that you do not use any funky fonts. Use the standard ones that are easy to read and clear. Keep with the standard 10pt font and don't overuse italics, bolds and underlines.

Most of all - when you go into the interview - be yourself. You are a lovely young woman with a great deal to offer. They would be very lucky to have you.