All of our 34 plants around the world are operating, and we are satisfying all of our customer needs.
Overall, our sales were up sequentially 28% from the second quarter and down 5% from the third quarter of last year on a pro forma basis.
First, as we look sequentially, the Americas saw the largest quarterly net sales improvement as sales grew 48% sequentially, driven primarily by stronger volumes.
A similar story I heard in Asia Pacific, EMEA, and our Global Specialty Businesses, where volume improvement drove net sales increases of 24%, 21% and 16% respectively compared to the second quarter.
Metalworking increased the most and grew 39% sequentially from the second quarter, due primarily to automotive OEMs and related suppliers coming back from the prolong shutdowns or significantly reduced production rates in the second quarter due to COVID.
Our other industry groups of metals and Global Specialty Businesses also increased and showed growth of 21% and 16% respectively.
So overall, our sequential volumes were up 27% but our pro forma volumes were still down versus last year by approximately 10%, when excluding the positive impact of Norman Hay, which we acquired last October.
This approximately 10% decline was felt in all regions and segments.
I also want to point out that we did continue to take market share despite the weakness in our end markets as our continued analysis shows that we have total organic sales growth due to net share gains of approximately 2% in this quarter versus the third quarter of last year.
As we said previously, we estimate it will take at least two more years for our markets to fully return, and some markets like aerospace, which makes up about 3% of our sales, will take more time than that.
This pandemic and its impacts have been similar in many ways to what we went through in late 2008.
Essentially, all discretionary expenses have been eliminated, we stopped new hires where possible, some positions were furloughed, and our planned capital expenditures have been cut by over 30%.
For 2020, our current estimate is $58 million of cost synergies achieved versus our earlier estimate of $53 million.
Also, the total synergies we estimate that we will achieve in 2021 have been raised from $65 million to $75 million with '22 reaching $80 million.
In this quarter, we achieved $17 million in synergies, and we expect sequential improvement during our future quarters.
Also, our cash flow is very strong and our net debt decreased by 7% or $58 million.
Looking ahead, we anticipate that throughout the next year or two, our markets will show gradual sequential improvement.
For the fourth quarter, we expect our adjusted EBITDA to be in the ballpark of the third quarter, and for the full year, we expect our adjusted EBITDA to exceed $215 million.
As we look forward to 2021, we expect our adjusted EBITDA to increase by 20%-plus as we continue our integration savings, take market share in the marketplace, and benefit from an expect gradual rebound in demand.
Remember that we closed the Combination on August 1, 2019, so our actual reported and non-GAAP Q3 2019 results include only two months of Houghton.
Please see slides 6 through 10 now, while I review some highlights.
As Mike noted, we saw sales rebound of $367 million in the third quarter, up 28% from $286 million in Q2, but still down 5% from pro forma Q3 2019 sales of $386 million, due primarily to lower volumes as a result of COVID-19.
Our gross margin of 38.2% is up significantly from our gross margin of 34% in Q2 and 32.3% in Q3 of last year, which we estimate would have been about 35.5%, excluding a purchase accounting adjustment.
We also saw a benefit to our gross margin this quarter that we estimated approximately 0.5%, which we attribute to price mix that we believe was unique to the quarter.
Our non-GAAP operating income of $43.2 million rebounded significantly from Q2's $11.2 million and is also up 25% from $34.5 million in Q3 of last year, primarily due to the addition of Houghton and Norman Hay and the benefits of realized cost synergies, partially offset by the negative impact of COVID-19.
Similarly, our non-GAAP earnings per share of $1.56 is up significantly from Q2's $0.21, while it is flat to last year as a result of the additional shares issued at close of the Combination.
Our effective tax rate in the current quarter was an expense of 8.1% versus a benefit of 27.6% in Q3 of last year.
Excluding the impact of all unusual items, we estimate that our Q3 effective tax rates would have been approximately 24% and 20% for 2020 and 2019, respectively.
For the full year, we expect our effective tax rate, excluding all unusual items will be in the range of 23% to 25%.
As Mike mentioned earlier, we're pleased to see our adjusted EBITDA almost double to approximately $64 million in Q3 from the Q2 level of approximately $32 million and increased approximately 5% compared to our pro forma adjusted EBITDA of $61 million in Q3 of last year.
Indeed, we saw this in both Q2 and Q3 and are pleased to report that our year-to-date operating cash flow more than tripled versus last year to $112 million.
In addition, our low capital intensity allows us flexibility with our capital spending, and consistent with our Q2 call, we're on pace to reduce our capex by more than 30% versus our original pre-COVID estimate.
As a result of our strong cash flow, we were able to reduce net debt by $58 million, a 7% reduction from Q2.
This improves our leverage ratio to 3.4 times from 3.7 times at the end of June, and our bank calculated leverage at the end of Q3 was about 2.9 times versus our covenant maximum of 4.25 times.
In addition, as Mike noted, we further increased our estimates of expected Combination cost synergies from $53 million to $58 million in 2020, from $65 million to $75 million in 2021, and from $75 million to $80 million in 2022.
In 2021, as Mike mentioned, we expect to see a greater than 20% increase in adjusted EBITDA.

Summary:
Looking ahead, we anticipate that throughout the next year or two, our markets will show gradual sequential improvement.
For the fourth quarter, we expect our adjusted EBITDA to be in the ballpark of the third quarter, and for the full year, we expect our adjusted EBITDA to exceed $215 million.
Similarly, our non-GAAP earnings per share of $1.56 is up significantly from Q2's $0.21, while it is flat to last year as a result of the additional shares issued at close of the Combination.