mSSD compatibility with Inspiron 15 and Inspiron 15R

Hi all, I am looking for some advice before I buy. Can anyone advise if  I can install an mSSD into the following laptops? I read somewhere some have a mSSD slot, others do not. These are some models I am looking at.

Dell Inspiron 15 3521

Dell Inspiron 15R 5537

Dell Inspiron 15R 7520

I really want to run either a seperate SSD, or more likely in a 15" laptop, an mSSD, in addition to main data storage HDD.

Any help appreciated. 



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