| | Three Forks, MT | Eureka, IL | United States |
| Population | 2,084 | 5,569 | 329,725,481 |
| Female Population | 50.3% | 53.8% | 50.5% |
| Male Population | 49.7% | 46.2% | 49.5% |
| Median Age | 41 | 38.1 | 38.4 |
| space3 | | | |
| Population - 2010 | 1,868 | 5,345 | 309,327,143 |
| Population - 2000 | 1,728 | 4,871 | 285,036,114 |
| Population - 1990 | 1,203 | 4,435 | 251,960,433 |
| Pop. 2000 to Now | 18.2% | 7.4% | 15.6% |
| Pop. 1990 to Now | 69.8% | 18.0% | 30.8% |
| space4 | | | |
| Population Density | 1,429.4 | 2,096.8 | 93.4 |
| Land Area | 1.5 | 2.7 | 3,531,905.4 |
| Water Area | 0.1 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Three Forks, MT | Eureka, IL | United States |
| Age 0 to 4 | 6.0% | 6.2% | 5.9% |
| Age 5 to 9 | 7.4% | 4.8% | 6.1% |
| Age 10 to 14 | 7.1% | 5.6% | 6.6% |
| Age 15 to 17 | 3.1% | 1.7% | 3.9% |
| Age 18 to 20 | 1.8% | 7.1% | 4.0% |
| Age 21 to 24 | 6.7% | 10.6% | 5.2% |
| Age 25 to 34 | 11.4% | 10.9% | 13.8% |
| Age 35 to 44 | 12.5% | 10.6% | 12.9% |
| Age 45 to 54 | 7.7% | 10.2% | 12.6% |
| Age 55 to 59 | 5.9% | 7.1% | 6.7% |
| Age 60 to 64 | 12.8% | 6.4% | 6.3% |
| Age 65 to 74 | 12.1% | 5.9% | 9.6% |
| Age 75 to 84 | 2.7% | 6.8% | 4.5% |
| Age 85 and over | 2.7% | 6.1% | 1.9% |
| space1 | | | |
| Race | | | |
| | Three Forks, MT | Eureka, IL | United States |
| White | 93.7% | 90.9% | 59.4% |
| Black | 0.0% | 1.5% | 12.2% |
| Asian | 3.2% | 0.1% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.0% | 0.4% |
| Two or More Races | 0.6% | 2.5% | 3.2% |
| Hispanic | 2.5% | 5.0% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Three Forks, MT | Eureka, IL | United States |
| Married Population | 59.2% | 56.2% | 49.8% |
| Currently Married | 58.5% | 55.7% | 48.0% |
| Married but Separated | 0.7% | 0.5% | 1.8% |
| Single Population | 40.8% | 43.8% | 50.2% |
| Never Married | 23.7% | 22.3% | 33.8% |
| Divorced | 9.7% | 11.8% | 10.8% |
| Widowed | 7.4% | 9.7% | 5.6% |
| space5 | | | |
| Household Size | 2.59 | 2.24 | 2.60 |
| Households | 799 | 2,211 | 124,010,992 |
| Family Households | 471 | 1,448 | 80,755,759 |
| Married couple, w/children | 36.7% | 40.9% | 30.5% |
| Married couple, no children | 56.3% | 47.9% | 42.9% |
| Non Family Households | 328 | 763 | 43,255,233 |
| Single householder, w/children | 3.0% | 6.1% | 15.8% |
| Single householder, no children | 4.0% | 5.1% | 10.8% |