Specifically, we delivered comp store sales growth of 3.1%, while sustaining an identical two-year stack of 13.3% compared with Q2.
In addition, we also delivered significant improvements in our adjusted gross margin rate of 246 basis points, led by our category management initiatives.
Our adjusted SG&A costs as a percentage of net sales were 209 basis points higher as we lapped a unique quarter in Q3 2020.
Overall, we delivered adjusted operating income margin expansion of 37 basis points to 10.4% versus Q3 2020.
Adjusted diluted earnings per share of $3.21 increased 21.6% compared with Q3 2020 and 31% compared with the same period of 2019.
Our year-to-date adjusted earnings per share are up approximately 50% compared with 2020.
Year-to-date, our balance sheet remains strong with a 19% increase in free cash flow to $734 million, while returning a record $953 million to our shareholders through a combination of share repurchases and quarterly cash dividend.
As we all know, over the last 18 months, the pandemic changed consumer behavior across our industry, which led to a surge in DIY omnichannel growth in 2020, while the Professional business declined.
We've implemented the dynamic assortment tool in all company-owned US stores as well as over 800 independent locations.
During Q3, we announced a multi-year agreement with our national customer Bridgestone to sell DieHard batteries in more than 2,200 tire and vehicle service centers across the United States.
In terms of our independent business, we added 16 net new independent Carquest stores in the quarter, bringing our total to 44 net new this year.
In Q3, we continue to leverage our Speed Perks loyalty program as VIP membership grew by 13% and our number of ELITE members, representing the highest tier of customer spend, increased 21%.
We successfully transitioned to our new WMS in approximately 36% of our distribution center network as measured by unit volume.
We remain on track with our sales and profit per store initiative, including our average sales per store objective of $1.8 million per store by 2023.
In terms of new locations year-to-date, we've opened 19 stores, six new WORLDPAC branches and converted 44 net new locations to the Carquest independent family.
This puts our net new locations at 69, including stores, branches and independents during the first three quarters.
Separately, we're actively working to convert the 109 locations in California we announced in April.
In Q3, we saw a 22% reduction in our total recordable injury rate compared with the prior year.
Our lost time injury rate improved 14% compared with the same period in 2020.
This made it even easier for customers to participate and helped us achieve a record-setting campaign of $1.7 million.
In Q3, our net sales increased 3.1% to $2.6 billion.
Adjusted gross profit margin improved 246 basis points to 46.2%, primarily the result of our ongoing category management initiatives, including strategic pricing, strategic sourcing, own brand expansion and favorable product mix.
In the quarter, same SKU inflation was approximately 3.6%, which was part of [Phonetic] our plan entering the year and was by far the largest headwind we had to overcome within gross profit.
Year-to-date, adjusted gross margin improved 184 basis points compared with the same period of 2020.
As a percent of net sales, our adjusted SG&A deleveraged by 209 basis points, driven primarily by labor costs, which included a meaningful cost per hour increase as well as higher incentive compensation compared to the prior year.
In addition, we incurred higher delivery expenses related to serving our Professional customers and approximately $10 million in start-up costs related to the conversion of our California locations in Q3.
Year-to-date, SG&A as a percent of net sales was relatively flat compared to the same period of 2020, increasing 9 basis points year-over-year.
While we've reduced our COVID-19-related costs by $13 million year-to-date, the health and safety of our team members and customers continues to be our top priority.
Our adjusted operating income increased to $274 million in Q3 compared to $256 million one year ago.
On a rate basis, our adjusted OI margin expanded by 37 basis points to 10.4%.
Finally, our adjusted diluted earnings per share increased 21.6% to $3.21 compared to $2.64 in Q3 of 2020.
Compared with 2019, adjusted diluted earnings per share was up 31% in the quarter.
Our free cash flow for the first nine months of the year was $734 million, an increase of 19% versus last year.
This increase was primarily driven by improvements in our operating income as well as our continued focus on working capital metrics, including our accounts payable ratio, which expanded 351 basis points versus Q3 2020.
Year-to-date through Q3, our capital investments were $191 million.
In Q3, we returned approximately $228 million to our shareholders through the repurchase of 1.1 million shares at an average price of $205.65.
Year-to-date, we've returned approximately $792 million to our shareholders through the repurchase of nearly 4.2 million shares at an average price of $189.43.
Since restarting our share repurchase program in Q3 of 2018, we returned over $2 billion in share repurchases at an average share price of approximately $164.
Additionally, we paid a cash dividend of $1 per share in the quarter totaling $63 million.
This included improved pricing and terms while also increasing the overall facility size to $1.2 billion.
This guidance incorporates continued top-line strength, ongoing inflationary headwinds and up to an additional $10 million in start-up costs in Q4 related to our West Coast expansion.
As a result, we're updating our full year 2021 guidance to net sales of $10.9 billion to $10.95 billion, comparable store sales of 9.5% to 10%, adjusted operating income margin rate of 9.4% to 9.5%, a minimum of 30 new stores this year, a minimum of $275 million in capex and a minimum of $725 million in free cash flow.

Summary:
Specifically, we delivered comp store sales growth of 3.1%, while sustaining an identical two-year stack of 13.3% compared with Q2.
Adjusted diluted earnings per share of $3.21 increased 21.6% compared with Q3 2020 and 31% compared with the same period of 2019.
In Q3, our net sales increased 3.1% to $2.6 billion.
Finally, our adjusted diluted earnings per share increased 21.6% to $3.21 compared to $2.64 in Q3 of 2020.
As a result, we're updating our full year 2021 guidance to net sales of $10.9 billion to $10.95 billion, comparable store sales of 9.5% to 10%, adjusted operating income margin rate of 9.4% to 9.5%, a minimum of 30 new stores this year, a minimum of $275 million in capex and a minimum of $725 million in free cash flow.