Supply-Side revenue grew $1.0 million, or 18%, in the three months ended March 31, 2021 as compared to the three months ended March 31, 2020, primarily driven by increased uptake of our solutions from our platform and publisher customers.
Cost of Revenue (exclusive of depreciation and amortization shown below)
Cost of revenue increased by $2.9 million, or 40%, from $7.3 million in the three months ended March 31, 2020 to $10.2 million in the three months ended March 31, 2021. The increase was primarily due to higher software and other technology costs to support our increased volumes, as well as higher partner costs from revenue-sharing arrangements with our Advertiser Programmatic partners.
Product Development Expenses
Product development expenses increased by $3.9 million, or 37%, from $10.3 million in the three months ended March 31, 2020 to $14.2 million in the three months ended March 31, 2021. The increase was primarily due to an increase in personnel costs of $3.4 million, which reflects our continued hiring of resources to support our product-development efforts.
Sales, Marketing and Customer Support Expenses
Sales, marketing and customer support expenses increased by $3.2 million, or 26%, from $12.3 million in the three months ended March 31, 2020 to $15.5 million in the three months ended March 31, 2021. The increase was primarily due to an increase in personnel costs of $4.5 million to support our sales efforts, build market presence in international markets, drive continued expansion with our existing customers, as well as support both existing and new customers, offset by a $1.3 million reduction in non-personnel costs. The decline in non-personnel expenses was due primarily to a $0.8 million reduction in sales tax expenses and a $0.5 million reduction in expenses for marketing, travel, and entertainment.
General and Administrative Expenses
General and administrative expenses increased by $1.1 million, or 11%, from $10.7 million in the three months ended March 31, 2020 to $11.8 million in the three months ended March 31, 2021. The increase was primarily due to a $3.0 million increase in compensation expenses offset by a reduction of non-compensation expenses of $1.9 million, which included a reduction of severance expense of $0.8 million and reduction of professional fees of $0.6 million.
Depreciation and Amortization
Depreciation and amortization increased by $1.2 million, or 19%, from $5.9 million in the three months ended March 31, 2020 to $7.1 million in the three months ended March 31, 2021. The increase was primarily due to an increase in depreciation related to capital expenditures.
Interest Expense
Interest expense is mainly related to our Prior Credit Facilities and New Revolving Credit Facility, which carry a variable interest rate. Interest expense decreased by $0.8 million, from $1.2 million in the three months ended March 31, 2020 to $0.4 million in the three months ended March 31, 2021. The decrease was attributable to a reduction in outstanding debt. In October 2020, we entered into a New Revolving Credit Facility and repaid all amounts outstanding under the Prior Credit Facilities.
Other (Income) Expense, Net
Other income decreased by $0.3 million, from income of $0.3 million in the three months ended March 31, 2020 to income of less than $0.1 million in the three months ended March 31, 2021, primarily due to a decrease in realized gains for changes in fair value related to contingent payments from our acquisition of Zentrick NV in February 2019, offset by a decrease in unrealized losses related to changes in exchange rates.