March 31, 2021 that are included in our results of operations for the three months ended March 31, 2022 include Prairie Capital Management, Rollins Financial, ARS Wealth Advisors, Badgley Phelps Wealth Managers, Ancora Holdings, Sonora Investment Management, Cardinal Point, Ullmann Wealth Partners, Mosaic Family Wealth, Alley Company, Cassaday & Company and Provident Financial Management. Additionally, our partner firms completed 24 acquisitions subsequent to the three months ended March 31, 2021. The new partner firms contributed approximately $54.2 million in revenue during the three months ended March 31, 2022. The balance of the increase of $86.1 million was due to the revenue growth at our existing partner firms associated with wealth management services, which includes partner firm-level acquisitions.
Other revenues increased $2.1 million, or 10.6%, for the three months ended March 31, 2022 compared to the three months ended March 31, 2021. Other revenues from new partner firms was approximately $0.7 million. The balance of the increase of $1.4 million was due primarily to an increase in recordkeeping and administration fees.
Operating Expenses
Compensation and related expenses increased $40.8 million, or 28.9%, for the three months ended March 31, 2022 compared to the three months ended March 31, 2021. The increase related to new partner firms was approximately $13.9 million. The balance of the increase of $26.9 million was due primarily to an increase in salaries and related expense due to the growth of existing partner firms and partner firm-level acquisitions offset in part by a decrease in non-cash equity compensation expense of $5.6 million.
Management fees increased $35.8 million, or 35.0%, for the three months ended March 31, 2022 compared to the three months ended March 31, 2021. The increase related to new partner firms was approximately $13.5 million. Management fees are variable and a function of earnings during the period. The balance of the increase of $22.3 million was primarily due to partner firm-level acquisitions and the increase in earnings during the three months ended March 31, 2022 compared to the three months ended March 31, 2021.
Selling, general and administrative expenses increased $24.8 million, or 38.9%, for the three months ended March 31, 2022 compared to the three months ended March 31, 2021. New partner firms added approximately $8.1 million. The balance of the increase of $16.7 million was due primarily to an increase in expenses related to travel and entertainment, information technology and professional fees related to the growth of our existing partner firms and partner firm-level acquisitions.
Intangible amortization increased $17.3 million, or 40.2%, for the three months ended March 31, 2022 compared to the three months ended March 31, 2021. The increase related to new partner firms was approximately $11.9 million. The balance of the increase of $5.4 million was due primarily to partner firm-level acquisitions.
Non-cash changes in fair value of estimated contingent consideration decreased $34.9 million for the three months ended March 31, 2022 compared to the three months ended March 31, 2021. During the three months ended March 31, 2022, the probability that certain contingent consideration payments would be achieved decreased due to Monte Carlo Simulation changes associated with market conditions and forecasts, resulting in a decrease in the fair value of the contingent consideration liability.
Other income (expense)
Interest expense increased $7.1 million, or 67.4%, for the three months ended March 31, 2022 compared to the three months ended March 31, 2021. The increase was due primarily to higher average outstanding borrowings during the three months ended March 31, 2022 compared to the three months ended March 31, 2021.
Income Tax Expense
Income tax expense increased $14.4 million for the three months ended March 31, 2022 compared to the three months ended March 31, 2021. For the three months ended March 31, 2022, we recorded tax expense based on an estimated annual effective tax rate of 28.6%. The estimated annual effective tax rate is primarily related to federal, state and local income taxes imposed on Focus Inc.’s allocable portion of taxable income from Focus LLC.