I can't tell about the Pro 7, but I had tried the same with my Pro 5 (or Pro 2017, since officially for whatever reason it was never named 'Pro 5'). While mechanically things were OK with the spacer (which I got for free after calling MS) I ran into issues with the power supply.
If you look carefully: the Pro 3 dock power supply has slightly less voltage and can provide less current only than the Pro 4-7 power supplies. So, while the device mechanically fitted well and worked OK in the Pro 3 dock, the battery often would not charge (sometimes not charge at all or not charge full) and esp. it typically did not charge while working on it (i.e. system active PLUS charging at the same time was too much). And the stronger supplies of the Pro4+ don't fit into the plug of the Pro 3 dock. :-(
Thus, after having left the house a few times with a half-empty battery due to this, I finally switched to the "brick" (as I call it), i.e. the newer docking station. Since then I can always rely on a full battery after having charged at least a few hours (or e.g over night).
Just for your consideration...
