How to calculate Real GDP from quarterly change How do I calcualte a countries Real GDP from QoQ changes?

Example - Australia
2018 - Real GDP was 1432195178668.32 $Billion
2019  Q1 was 0.5%
2019  Q2 was 0.6%
2019  Q3 was 0.4%

How do I calculate real GDP on Q3?
Assuming 1432195178668.32 was the last value in Q4 2018 you got, just calculate 1432195178668.32 * (1.005) to get the 2019 Q1 Value, save the value. Then take 2019 Q1 * (1.006) to get the next value etc. Easy to do via R or Excel.

Basically if you had an increase x just multiply by (1+x) where x is in decimals, so a change of -0.75 % becomes * (1 - 0.0075)

Then, simply take the average of the four quarters if you want the annual value:
$$ \frac{(Q1.2019 + Q2.2019 + Q3.2019 + Q4.2019)} {4} $$