Hi Bill, it will all depend on the coal.. Bituminous coal has several characteristics that make it more of a challenge to burn compared to Anthracite.. Most Anthracite burns just about like any other anthracite, but Bituminous coal's burning characteristics will vary from location to location and mine to mine..
Bituminous coal gives off a lot of sooty smoke and it often is very smelly. Depends on the Volitiles level and the Sulphur content.
Bituminous coal often swells when it is heated, the coal sticking together into a solid sheet of coal, this sheet of coal will block off the airflow through the coal, and limit the heat output from the fire.. This sheet often will bridge over the entire firebox, and the fire will burn out below the 'bridge' of coal..
Anthracite coal's pieces stay individual, and burn like a bunch of red-hot marbles..
When I was burning Bituminous coal in my hand fired boiler, I had to return to the fire after an hour and break up the stuck-together coal, so the air could get through the coal bed.. I had to keep an open flame in a corner of the fire, the volitiles from Bit. coal will, for sure ignite and create a very dirty, sooty, 'puff-back' when they ignite.. I blew my flue pipes off several times.. even with 3-4 screws in each joint..
So the answer is yes you can burn Bituminous coal in your Mark IIi, but I wouldn't buy very much coal,, because I doubt that you will like the burning characteristics or soot.. If Bitum. coal is the only choice, then you CAN make it work, but it is not anywhere as easy or trouble-free to burn as anthracite coal.
Best of luck,, Greg L