Admirals Aokiji + Akainu + Kizaru would've been overkill, they would've destroyed Roger Pirates + Rocks Pirates.
You can throw Garp & his fleet to help Roger and Rocks and they still would've been destroyed.
The Valley incident was 38 years ago and even 14 years ago Sengoku was still an Admiral ( read a bit of Law's flashback in 763 & 764 )
Here is Admiral Sengoku 14 years ago :
https://mangasee123.com/read-online/One-Piece-chapter-764-page-21.html
So Kuzan Akainu Kizaru probably weren't Admirals yet.
With Kuzan's words "two men who came up the ranks together", referring himself & Akainu :
https://mangasee123.com/read-online/One-Piece-chapter-1081-page-8.html
I'm thinking he Akainu and Kizaru became Vice-Admirals & Admirals together/exactly at the same time, 14/13 years ago at the earliest.
Is Zephyr canon ?
I'm not sure Marines had another top tier alongside Kong Sengoku Garp.
Maybe i'm willing to assume there was 1 more Admiral besides Sengoku, but 2 ? Nope. Garp was basically one of the Admirals, so just another top-tier would've been enough to make it the 3 Admirals.