. A polymeric powder composition for three-dimensional printing, the polymeric powder composition comprising: a first polymer particle of a first average size; a second polymer particle of a second average size; and a third polymer particle of a third average size; wherein: each of the first average size, the second average size, and the third average size independently ranges from 5 μm to about 100 μm; and a sum of fractional weight ratios of all polymer particles in the polymeric powder composition equals 1; and characterized in that the first polymer particle of a first average size is present in an amount ranging from about 70 wt% to about 95 wt%; the second polymer particle of a second average size is present in an amount ranging from about 0.5 wt% to about 21 wt%, the second average size being smaller than the first average size; and the third polymer particle of a third average size is present in an amount ranging from greater than 0 wt% up to about 21 wt%, the third average size being smaller than the second average size; and wherein the polymeric powder composition has a computationally predetermined packing density range from about 0.4 g/ml to about 0.9 g/ml, wherein the computationally predetermined packing density is determined by a computational modeling method in which a computer receives at least the structure of the polymers that are to be used for the particles and the desired particle size distribution as input, and delivers at least the computationally predetermined packing density of a simulated layer formed with a polymeric particle composition that corresponds to the input polymer and size distribution information. a first polymer particle of a first average size; a second polymer particle of a second average size; and a third polymer particle of a third average size; wherein: each of the first average size, the second average size, and the third average size independently ranges from 5 μm to about 100 μm; and a sum of fractional weight ratios of all polymer particles in the polymeric powder composition equals 1; and characterized in that the first polymer particle of a first average size is present in an amount ranging from about 70 wt% to about 95 wt%; the second polymer particle of a second average size is present in an amount ranging from about 0.5 wt% to about 21 wt%, the second average size being smaller than the first average size; and the third polymer particle of a third average size is present in an amount ranging from greater than 0 wt% up to about 21 wt%, the third average size being smaller than the second average size; and wherein the polymeric powder composition has a computationally predetermined packing density range from about 0.4 g/ml to about 0.9 g/ml, wherein the computationally predetermined packing density is determined by a computational modeling method in which a computer receives at least the structure of the polymers that are to be used for the particles and the desired particle size distribution as input, and delivers at least the computationally predetermined packing density of a simulated layer formed with a polymeric particle composition that corresponds to the input polymer and size distribution information. each of the first average size, the second average size, and the third average size independently ranges from 5 μm to about 100 μm; and a sum of fractional weight ratios of all polymer particles in the polymeric powder composition equals 1; and characterized in that the first polymer particle of a first average size is present in an amount ranging from about 70 wt% to about 95 wt%; the second polymer particle of a second average size is present in an amount ranging from about 0.5 wt% to about 21 wt%, the second average size being smaller than the first average size; and the third polymer particle of a third average size is present in an amount ranging from greater than 0 wt% up to about 21 wt%, the third average size being smaller than the second average size; and wherein the polymeric powder composition has a computationally predetermined packing density range from about 0.4 g/ml to about 0.9 g/ml, wherein the computationally predetermined packing density is determined by a computational modeling method in which a computer receives at least the structure of the polymers that are to be used for the particles and the desired particle size distribution as input, and delivers at least the computationally predetermined packing density of a simulated layer formed with a polymeric particle composition that corresponds to the input polymer and size distribution information.