I think that in the boiler rooms there were: 6 engineers, 3 on each of 2 watches 159 firemen, 53 on each of 3 watches. That provides one man per boiler front. 72 trimmers, 24 on each of 3 watches gives 2 supporting each row of boiler fronts. Maybe they were on 2 watches and there were 3 in each row.
That gives us: -Engineers directing the starting or finishing of boilers, watch the warm up rates. See that firing rates are followed, fires are even over the grate (holes allow cold air through), fires are clean burning with smoke (waste) minimized, water levels are okay and draft fans are adjusted, scum cocks or blow downs operated regularly. They would also oversee fixing such problems as grates burning through or boiler tubes leaking.
-Stokers shoveling in the coal with an even spread, raking it and breaking up clinkers, pulling the ashes out of the pit under the grate. There would be a repetitive pattern to the 3 tasks over the 3 furnaces. Sledge hammers might be used for breaking up large chunks of coal. I think the stokers are also the ones who move the ashes to the ash ejectors and dispose of them, wheelbarrows might be used for this. Each man would handle about 2000 pounds of coal per hour. The stokers also watch the water glass and adjust the feedwater valve.
-Trimmers moving coal out of the side bunkers, knocking down piles and moving it to the door in front of the boiler where it is within reach of the stoker.