[2020] Item share progression across 3 platforms?

Hey there!

Yes! Set your primary to Steam, connect both your PS4 and Series S, and your inventory from each platform should be usable. There are a few exceptions to this, those exceptions can be seen here https://support.rocketleague.com/hc/en-us/articles/360053544774-Which-items-can-be-shared-across-linked-accounts-