With Construct 3's native support for Spacing-data, there is no need to use SprintFont+ anymore, or have it migrated to C3. The following tutorial will outline how to revert your C2 project back to the Scirra Spritefont plugin, so you can open it in C3, and just add the spacing data back in.
I recommend you edit a few things in your C2 project first, to make conversion easier. If you actually use "SprintFont" in your object name, change it to something else for now. You can always change it back, but it will be less confusing when editing the CAPROJ file later, if there are no extra SprintFont names in there.
Ideally you should use Give-Your-Fonts-Mono to regenerate your data files, since you need the new C3 format it generates for the conversion. (GYFM has changed over time, so you may need to tweak a few things).
Add a default/Scirra Spritefont to your project. We will use this as our new template for the conversion. Again, you can always remove it after.
1) Use "Save As Project" so we can get to the internals of your project.
2) Go to that location, and open the CAPROJ file in a text editor.
3) Find the SpriteFontPlus plugin line near the top of the file.
4) Remove the SpriteFontPlus line completely!
5) Replace all references to SpriteFontPlus (you'll see them further down in your project, wherever they are used) with Spritefont2.
8) Open this edited CAPROJ file in C2. You'll see the Spritefonts are all the default version.
9) Save As Single File.
10) Open project in C3.
11) For each Spritefont object in the project, copy the C3 spacing-data text from the 'Construct 3 Only' section of the generator file and paste it into the 'Spacing data' field of the Spritefont.
Again, things have changed, so you may need to tweak the spacing a bit, but the changes should be relatively minor. You now have official C3 Spritefont support without the need for the Sprite Font Plus third-party plugin.
I've attached a sample CAPX that you can play with first, to test the process yourself, without mucking up your own project.
Sprite Font Generator - Give Your Fonts Mono