Hi Samantha,
They should be viable to some extent. Exposure to high heat and/or humidity lowers germination rates. Time also lowers germination rates, but 3 years isn't that long for those seeds. So if you had them stored in halfway decent conditions these past few years, you will get germination.
Remy

An update, many months later!!

I winter-sowed a bunch of them back in January and most of them germinated! I was shocked and thrilled! :)

The campion didn't flower this year, but the pink cone flower and black eyed susans produced a few plants that have flowers on them right now.

Yay for my first experience with winter sowing!