Disclaimer: this will be wonky, probably you will not like it, but it’s free, so what do you have to lose.
Facts: Dropbox and OneDrive are both free; Dropbox allows only up to 3 paired devices at all times (no idea about OneDrive).
Let’s say you want to let a fourth device access Dropbox for free, at least (or even better yet) for just a subset of files in your dropbox.
Install and configure it, then make use of Windows
mklink command to create symlinks, by opening an elevated command prompt from the start menu (open start menu, type “cmd” and right click on the command prompt link, choosing “Run as administrator”), then
cd into OneDrive’s root folder, and type:
mklink /D things path\to\Dropbox\folder\you_want_to_share
from now on, OneDrive will start uploading the contents of the Dropbox folder you specified, so they will be available to your other PCs’ OneDrive installations past the limit of 3 concurrent devices.
I tested that an empty text file created in the 4th PC’s OneDrive folder will appear in the Dropbox+OneDrive PC’s Dropbox folder, but I noticed that edits of this empty file don’t really propagate… maybe it’s just a matter of time? Also, deleting that file in the Dropbox+OneDrive PC will not delete the file in the 4th OneDrive only PC.
This is just from a few minutes of testing, I wanted to share the news.