Rob nailed it! Begin with the baseboards. I believe that the output rating is typically calculated at 550 BTU's per lineal foot for hot water baseboards, assuming a 20 degree drop from 180 degrees to 160 degrees. If that is accurate, then the three houses would need a combined total of a bit over 900 feet of baseboards in order to accept a full output of 500,000 BTU's.
That said, coal boilers fire at a peak of about 80% efficiency, so a boiler with an honest max input of 500K BTU's would have a maximum potential output of about 400K BTU's. That output level can be handled (delivered to the homes) by about 725 feet of hot water baseboards. That's still a whopper load of baseboards.
If you assume that each home actually needs only about 85,000 BTU's worth of furnace input, then an AA 260 or AHS S260 should be plenty big. 260,000/3 = ~87,000 available intput BTU's per home (assuming here for simplicity that all 3 are quite similarly sized).
87,000 x 80% = ~70,000 output BTU's per home for 3 houses. Each home would require an average of 127 feet of baseboards in this case (totaling to ~380 feet), and this seems far more realistic. As Rob stated above, the biggest unknown will be the underground heat loss. You will also need some appreciable reserve for DHW if the boiler also supplies each of the homes hot water for showers, laundry, etc... A heat loss calculation is the only sure means to calculate all of this.
On an average winter day/night, probably about 15-20% of maximum rated output suffices to keep a home warm. The maximum output is only "potentially" needed when it is sub zero outside.