Take a look at the accepted answer to this question.
I will highlight a couple key points I think apply to you, and hopefully it makes sense.
The Playstation Plus subscription is tied to the SEN account, but that account can specify a single PS4 console as its primary PS4, which enables other (non-PS+) accounts to access everything on the console.
So any account using that console (the one specified as the primary by at least 1 PS+ subsriber) can enjoy the benefits of PS+, when using that console. That includes multiplayer.
Meaning you can have non-PS+ accounts utilizing the PS+ subscription if playing on the same console, meaning you cannot also be playing (even I was not aware of this...).
Keep in mind this answer was from Dec 2013, so not all information may be 100% accurate currently, but I believe most should be.
However, I can tell you that multiple consoles cannot all play online using the same PS+ subscription, so I suppose the TL;DR answer is "You will have to buy separate subscriptions if you're all playing online on different consoles at the same time."
You're essentially wanting to use the same service 3 times simultaneously while paying the price for 1.