This Is Probably A Silly Question, But...

[Rediahs]

I have these shells that I picked from the beach a few years ago. They've been dry this whole time. I thought they might spice up my fish's tank (it's a betta). I was wondering - could they be carrying anything that might hurt my fish? Or is it safe to just plunk them in? Remember they've been sitting in a container for at least 3 years, dry.

 

[Luke_e]

You could boil them for a while, this would remove most nasties, but there may still be a chance of a salt trace.
Hopefully someone else can confirm or deny this.

 

[Bignose]

What we really need to know is the stats of your water. Shells are largely calcium, which will dissolve if your water is soft. And, you may not want harder, most likely higher pH water than what you have now.

So, please get us the current pH, GH, and KH of you current water, and then we can probably advise you better.

As far as anything living on them, three years is more than long enough to be safe that nothing is on them. Fish diseases cannot live outside of water for anywhere near that long.