If you have adequate draw with the current flue pipe setup, even though it has three elbows, I'd live with it.. every time you cut another hole for a thimble in a chimney, you weaken it, risk cracking the masonry in ways you did not intend.. right now, with all three openings on one side, you have two sides and a back of the chimney 'stack' that is untouched and is supporting the weight of the masonry above.. if you cut into one of the sides, you have posibly compromised the strength of the chimney..
When burning coal, you don't have the issues of creostote, which probably was a problem with the three elbows in the flue pipe.. With coal, you just need a clean pipe with an uphill run from the burning appliance to the chimney..
I'd get the chimney cleaned, so it will draw as good as it can, put in new or clean flue pipe and elbows, and use it that way..
Now,, you mention three thimbles in the chimney right now,, are the other two capped or in use with other appliances?? If in use, these items are stealling draft from your wood/coal appliance... and they offer a way for fresh air to get to a possible chimney fire, making a chimney fire uncontrolable.. You don't have any fear of a chimney fire with coal, but you do need a steady reliable draft to keep coal burning steady..
Hope this helps.. Greg L