```js
class UnitConverter {
  // Conversion lookup tables mapped to base SI units:
  // Mass -> Grams
  // Length -> Meters
  // Time -> Seconds
  static MASS_IN_GRAMS = {
    // Base / SI
    g: 1.0, gram: 1.0, grams: 1.0,
    kg: 1000.0, kilogram: 1000.0, kilograms: 1000.0,
    mg: 0.001, milligram: 0.001, milligrams: 0.001,
    ug: 1e-6, mcg: 1e-6, microgram: 1e-6, micrograms: 1e-6,
    // Imperial / Customary
    oz: 28.349523125, ounce: 28.349523125, ounces: 28.349523125,
    lb: 453.59237, lbs: 453.59237, pound: 453.59237, pounds: 453.59237,
    st: 6350.29318, stone: 6350.29318,
    ton: 1016046.9088, tons: 1016046.9088,
    us_ton: 907184.74,
    tonne: 1000000.0, metric_ton: 1000000.0,
  };

  static LENGTH_IN_METERS = {
    // Base / SI
    m: 1.0, meter: 1.0, meters: 1.0, metre: 1.0, metres: 1.0,
    km: 1000.0, kilometer: 1000.0, kilometers: 1000.0,
    cm: 0.01, centimeter: 0.01, centimeters: 0.01,
    mm: 0.001, millimeter: 0.001, millimeters: 0.001,
    um: 1e-6, micron: 1e-6, micrometer: 1e-6,
    nm: 1e-9, nanometer: 1e-9,
    // Imperial / Customary
    in: 0.0254, inch: 0.0254, inches: 0.0254,
    ft: 0.3048, foot: 0.3048, feet: 0.3048,
    yd: 0.9144, yard: 0.9144, yards: 0.9144,
    mi: 1609.344, mile: 1609.344, miles: 1609.344,
    furlong: 201.168, furlongs: 201.168,
    chain: 20.1168, chains: 20.1168,
    nmi: 1852.0, nautical_mile: 1852.0,
    // Astronomical
    au: 149597870700.0, astronomical_unit: 149597870700.0,
    ly: 9460730472580800.0, light_year: 9460730472580800.0, lightyears: 9460730472580800.0,
    pc: 3.0856775814913673e16, parsec: 3.0856775814913673e16,
  };

  static TIME_IN_SECONDS = {
    // SI Units
    s: 1.0, sec: 1.0, second: 1.0, seconds: 1.0,
    ms: 0.001, millisecond: 0.001, milliseconds: 0.001,
    us: 1e-6, microsecond: 1e-6, microseconds: 1e-6,
    ns: 1e-9, nanosecond: 1e-9, nanoseconds: 1e-9,
    ps: 1e-12, picosecond: 1e-12, picoseconds: 1e-12,
    // Clock / Calendar
    min: 60.0, minute: 60.0, minutes: 60.0,
    h: 3600.0, hr: 3600.0, hrs: 3600.0, hour: 3600.0, hours: 3600.0,
    d: 86400.0, day: 86400.0, days: 86400.0,
    wk: 604800.0, week: 604800.0, weeks: 604800.0,
    fortnight: 1209600.0, fortnights: 1209600.0,
    mo: 2629746.0, month: 2629746.0, months: 2629746.0,
    yr: 31556952.0, year: 31556952.0, years: 31556952.0,
    decade: 315569520.0, decades: 315569520.0,
    century: 3155695200.0, centuries: 3155695200.0,
  };

  /**
   * Internal generic converter.
   * Converts a object of source unit/value pairs to a base SI value,
   * then scales to target unit.
   */
  static _convert(table, targetUnit, inputs) {
    const targetKey = String(targetUnit).toLowerCase();
    if (!(targetKey in table)) {
      throw new Error(`Unknown target unit '${targetUnit}'.`);
    }

    const inputKeys = Object.keys(inputs);
    if (inputKeys.length === 0) {
      throw new Error("Must supply at least one input unit value.");
    }

    let baseTotal = 0;
    for (const key of inputKeys) {
      const unitKey = String(key).toLowerCase();
      if (!(unitKey in table)) {
        throw new Error(`Unknown input unit '${key}'.`);
      }
      baseTotal += inputs[key] * table[unitKey];
    }

    return baseTotal / table[targetKey];
  }

  /**
   * Converts mass inputs to target unit (default SI: 'kg').
   * @param {string} unit Target unit
   * @param {Object} inputs Object map of input unit/values (e.g. { kg: 3 })
   */
  static toMassUnit(unit = "kg", inputs = {}) {
    return this._convert(this.MASS_IN_GRAMS, unit, inputs);
  }

  /**
   * Converts length inputs to target unit (default SI: 'm').
   * @param {string} unit Target unit
   * @param {Object} inputs Object map of input unit/values (e.g. { ly: 1 })
   */
  static toLengthUnit(unit = "m", inputs = {}) {
    return this._convert(this.LENGTH_IN_METERS, unit, inputs);
  }

  /**
   * Converts time inputs to target unit (default SI: 's').
   * @param {string} unit Target unit
   * @param {Object} inputs Object map of input unit/values (e.g. { min: 1, sec: 30 })
   */
  static toTimeUnit(unit = "s", inputs = {}) {
    return this._convert(this.TIME_IN_SECONDS, unit, inputs);
  }
}
```
```js
// Mass (default target: "kg")
console.log(UnitConverter.toMassUnit("oz", { kg: 3 }));
// Output: 105.82188584874129

console.log(UnitConverter.toMassUnit("kg", { lb: 10, oz: 5 }));
// Output: 4.677669815625

// Length (default target: "m")
console.log(UnitConverter.toLengthUnit("km", { ly: 1 }));
// Output: 9460730472580.8

console.log(UnitConverter.toLengthUnit("in", { furlongs: 4 }));
// Output: 31680

// Time (default target: "s")
console.log(UnitConverter.toTimeUnit("ms", { min: 1, sec: 30 }));
// Output: 90000

console.log(UnitConverter.toTimeUnit("days", { hours: 36, min: 120 }));
// Output: 1.5833333333333333

console.log(UnitConverter.toTimeUnit("s", { weeks: 2, hours: 12 }));
// Output: 1252800
```