3 swivel barrel muzzle loader rifle

Gonna need a lot more details than that. It could be anything. It could be a rare 400yr old hand crafted marvel worth hundreds of thousands, or it could be some monstrosity somebody cobbled together int heir basement 10yrs ago and worth nothing. Any markings present and pics would be a huge help.

Flayderman's shows several guns of the type you describe (some rifle-only, some combination rifle/shotguns) but to figure out what you've got exactly is going to take some more information and likely, some good pictures.

Some photos from the OP and some more information; I've found out that Elijah K. Purdy was a gunsmith in Schoolcraft, Michigan from 1868 to 1871, and that he also patented a "floor clamp" for squeezing floorboards down onto the joists before nailing, to eliminate squeaky floors. Anyone else with any info?
The pictures show it's in fairly nice shape, with a decent piece of wood for the buttstock, and the rolled-up paper in one of the bores is a receipt from a past purchaser.

Some more pics.

Similar rifles in the latest Flaydermans show for anywhere between $1000 and $7000, depending on the specific features, and that can go up depending on provenance, but that sort of valuation is going to take a hands-on appraisal by a professional.