I tried connecting the backs of the shelves with cardboard/foam poster boards I got at the Dollar Tree.

I hammered them onto the backs of the bookcases with small nails.

Then I attempted to fill in the gaps and nail holes with spackle.

Things went south when I attempted to paint it though to match my walls — it just completely unearthed itself from my makeshift wall (see images below) and became warped.

https://imgur.com/gallery/Sxp8SG0

At this point, it's pretty clear that the poster board idea failed me. I'll have to take them all off and try again from scratch.

What should I use instead to make the backs of these 3 bookcases appear connected as a singular wall? And that I can then paint to make it look a bit less like an eyesore?

As you may have guessed, money is right right now, so I'm trying to go for the cheapest option that will work.

So, any ideas fellow DIYers?