It’s hard to answer if you’re posting too much without seeing the posts. Are all 3 daily posts interesting, entertaining or inspiring? Do they bring value to your audiences time on social? It’s unlikely because that would take a great deal of resource and investment, and if the posts were great reach and engagement wouldn’t be dwindling.
The answer to “how often should I post?” is always – as often as you can post high-quality, engaging content that your audience will enjoy consuming and interacting with.
A note on each platform too. LinkedIn posts typically have a longer lifespan so you can afford to post less. Posting multiple times a day on LinkedIn usually results in you competing with yourself and cannibalising your own engagement.
Twitter is super fast paced so you can afford to be a little more active over there.
Instagram and Facebook sit somewhere in the middle.
TL;DR: focus on quality, not quantity. Only share when you’re sharing content that brings value to your audience (entertain, educate or inspire).