problems with SDCard after partitioning
As it turned out, your partitioning was due to the fact you wanted to use adoptable-storage (and not, as one might assume, to use link2sd). As our adoptable-storage tag-wiki points out, adopted storage will be encrypted – not just the files on it, but the entire partition. That explains two things:


attaching the card to your PC using a card reader, the PC cannot read the partition without having the encryption details. As you didn't provide them to the PC, it could not "open" it
attaching the device via MTP however shows the partition fine on your PC, as the Android device has decrypted it


Remains the mystic 3rd partition. The small size suggests it's some "control partition". A good guess as it turns out: if you list the label, you will find it's called android_meta (while your adopted partition will be labeled android_expand) – which you can find out e.g. by a Google search for "sm partition disk" 16m. Quoting from this Oopsmonk article on how adoptable storage creation works:


  It will create two partitions (android_meta and android_expand) for adoptable storage, the android_meta is a reserved space for feature use and android_expand is the external storage which is encrypted via dm-crypt.


With that last detail, your puzzle should be complete