The doting mother, 33, marked the occasion with three adorable family photos on her Instagram account, one of which saw August cuddling his baby brother Ernest whilst both in their mother's arms.
Captioning the heartwarming snaps, Eugenie lovingly wrote: "Happy 3rd Birthday to our dearest Augie. Forever a force of nature and forever giggling with you."
The featured image showed August giggling away with his mother whilst donning a vibrant pair of yellow Wellington boots. The mother-son pair sat on the floor wrapped up in cosy coats and woolly hats.
The second photo showed Eugenie giving her eldest a kiss whilst holding a hot dog during a family day out.
'Proud gran' Sarah, Duchess of York, was quick to weigh in on her grandson's big day. She reshared the sweet post from her daughter to her own Instagram Stories and wrote: "Happy Birthday to my wonderful Auggie. I love you so much #proudgran."
August was born on 9 February 2021 at the Portland Hospital in London, the same birthplace as his mother and many other royal children.
Eugenie and her husband-of-five-years Jack Brooksbank, took to social media shortly after his birth with an adorable family photo.
The proud parents couldn't have looked happier in the photo and were beaming from ear to ear.
Alongside the image were the words: "We wanted to introduce you to August Philip Hawke Brooksbank. Thank you for so many wonderful messages. Our hearts are full of love for this little human, words can't express. We are excited to be able to share these photos with you."
The couple welcomed their second child, Ernest on 30 May 2023. August and Ernest have the sweetest sibling bond and have done so from the moment they met.
Marking her little one's arrival, Eugenie posted an adorable photo of her newborn whilst big brother August cooed over him and stroked his head - so sweet!
"Jack and I wanted to share the news that we had our little boy, Ernest George Ronnie Brooksbank on 30th May 2023 at 8.49 weighing 7.1lbs," Eugenie penned in the caption. "He is named after his great great great Grandfather George, his Grandpa George and my Grandpa Ronald.
The Princess sweetly added: "Augie is loving being a big brother already."