WOW, just 3 days? If you're moving down the coast, then I'd suggest stopping in either Portofino or Portovenere as they are quite similar... and then go directly to Florence. You could sleep one night in Liguria and then make your way to Florence for the other 2 nights, doing Siena the second day. It will be trying to see a lot in a short amount of time, but hopefully you can return for a more relaxed and longer trip??