MEDIQ Financial Services, InterOcean Capital, Seasons of Advice, CornerStone Partners, Fairway Wealth Management, Kavar Capital Partners and Hill Investment Group. Additionally, our partner firms completed 15 acquisitions subsequent to the three months ended March 31, 2020. The new partner firms contributed approximately $14.3 million in revenue during the three months ended March 31, 2021. The balance of the increase of $41.9 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 $0.9 million, or 4.8%, for the three months ended March 31, 2021 compared to the three months ended March 31, 2020. Other revenues from new partner firms was approximately $0.4 million. The balance of the increase of $0.5 million was due primarily to an increase in recordkeeping and administration fees.
Operating Expenses
Compensation and related expenses increased $23.2 million, or 19.7%, for the three months ended March 31, 2021 compared to the three months ended March 31, 2020. The increase related to new partner firms was approximately $2.4 million. Non-cash equity compensation increased $7.3 million primarily associated with incentive unit grants and the modification of certain incentive units (see Note 9 to the unaudited condensed consolidated financial statements). The balance of the increase of $13.5 million was due primarily to an increase in salaries and related expense due to partner firm-level acquisitions and the growth of existing partner firms.
Management fees increased $18.4 million, or 22.0%, for the three months ended March 31, 2021 compared to the three months ended March 31, 2020. The increase related to new partner firms was approximately $4.7 million. Management fees are variable and a function of earnings during the period. The balance of the increase of $13.7 million was primarily due to the increase in earnings during the three months ended March 31, 2021 compared to the three months ended March 31, 2020 and partner firm-level acquisitions.
Selling, general and administrative expenses increased $1.2 million, or 2.0%, for the three months ended March 31, 2021 compared to the three months ended March 31, 2020. New partner firms added approximately $2.2 million. The increase from new partner firms was offset by a decrease of $1.0 million due primarily to a decrease in travel and entertainment, business development and acquisition activity due to the impact of Covid-19 on our business.
Intangible amortization increased $7.3 million, or 20.3%, for the three months ended March 31, 2021 compared to the three months ended March 31, 2020. The increase related to new partner firms was approximately $2.7 million. The balance of the increase of $4.6 million was due primarily to partner firm-level acquisitions.
Non-cash changes in fair value of estimated contingent consideration increased $57.3 million, or 182.7%, for the three months ended March 31, 2021 compared to the three months ended March 31, 2020. During the three months ended March 31, 2021, the probability that certain contingent consideration payments would be achieved increased due to Monte Carlo Simulation changes associated with market conditions, resulting in an increase in the fair value of the contingent consideration liability.
Depreciation and other amortization expense increased $0.6 million, or 21.0%, for the three months ended March 31, 2021 compared to the three months ended March 31, 2020.
Other income (expense)
Interest expense decreased $3.1 million, or 22.6%, for the three months ended March 31, 2021 compared to the three months ended March 31, 2020. The decrease was due primarily to lower average interest rates on outstanding borrowings during the three months ended March 31, 2021.
During the three months ended March 31, 2020, a loss on extinguishment of borrowings of $6.1 million was recognized in connection with the January 2020 Credit Facility amendment.