Well, I have split Coopers 6 gal refills among 3 LBKs several times with no problem.
That dark ale makes a nice beer like a pub "mild".
The 7g of Coopers yeast will do it if it is fresh. What I have done is to try it and see if I get some fermentation in 24 hours. If not, then add more yeast. If it starts I would leave it alone. When I have had old yeast, I just used more of it for one LBK and other yeast in the other LBKs. I did not have that issue with the Dark Ale though. Be prepared for it not to be as as crazy active as the Mr Beer yeast though - it is still OK.
Likely you will be OK. I would just split it & see. If the yeast takes longer to get going or is "underpitched" you will get more yeast added flavors which should be OK in a brown beer.
Using the Brew enhancer 2 instead of the 3 will result in the beer being just a touch less malty I believe but should be good. I did not have the brew enhancer so I used dried Pilsen malt and booster in the equivalent proportions. If came out fine.
You may find the yeast easier to split if you mix it with water first. Maybe some amount easy to divide by 3 like 1.5 cups then add 1 tsp sugar to the water and then the yeast. I would use boiled water cooled to room temp. and put it in a measuring jug and cover it. After a couple of hours or more you should see some bubbles forming. Then stir it up to mix evenly and have none at the bottom and then divide equally among the LBKs. You may find after adding all the water, malt, etc. to the LBKs they are still a bit warm, so you would need to wait until they are cooler anyway to add the yeast (like below 80 deg.). The Cooper's instructions say to add yeast if wort is below 90 deg, but better to let it cool more. And the LBKs cool pretty fast.
That recipe will make a beer around 3.5% which is a nice mild dark session beer but not particularly strong tasting. I actually made my 3 LBKs different recipes by making additions to 2 of them and leaving one standard. Doing again, (I have 2 more I got on sale) I would make all standard. My additions were stronger and more hoppy but I liked the original more. I have more to do and will likely make them standard recipe.
If you want it stronger add 8 oz Dried malt for added 1% ABV. Light malt is OK or you can use booster too if you have that.
Hops are a bit more tricky, as they can really change the flavor and may conflict with the "mild beer" flavor. Still, it depends what you like :-D.
What I did was not to make all of it in 1 pot, but I first used about 6 cups boiling water to dissolve the HME then split that between the LBKs using a ladle after putting the 4 qts. cold water in each. (careful not to drip over everything). I figured it would be tough to measure evenly from the can - lol. Then I dissolved the dried malt (or brew enhancer) but I did this for 1 LBK at a time weighing it out to make it easier to manage and dissolving each batch before adding to its LBK. Of course if you are using 1 packet and dividing equally you may not need to weigh, you can divide using measuring cups. Then also if you are making each LBK different you can customize at this stage.
If you like these Cooper's refills, you may find getting one of the MR B 6 Gal fermenters easier, although I personally can't manage one that big. The 2 gal ones are just so handy to move about..