If you tie you line to the shackle then an anchor hitch would be a good knot, plus I'd use some whipping twine and affix the working end to the line to make sure it cannot pull loose. The anchor bend self-tightens and is quite secure.
On the other hand, I like to splice and would splice my 3-strand into the chain or the shackle, since that would eliminate even more chance of chafe and a splice is stronger than a knot.
Anchor hitch: Anchor Hitch | How to tie the Anchor Hitch (Fisherman's Hitch) | Boating Knots
3-Strand to chain: http://www.samsonrope.com/site_files..._RopeChain.pdf
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