Best drive option

I ordered a new workstation with an internal ssl drive. I will put Platinum on that drive. I also want to add another one or two. My question is what is the best route, a microsd card, USB 3.0 flash drive, or external HD.

If it's a desktop/tower style workstation with room for additional SATA HDD's, I would throw in at least one 7200 RPM HDD at 1TB capacity or more, depending on your storage needs. That's the most practical and budget friendly way to go.
 
I load my Windows, Sonar and all other applications, plus all VST plugins onto my SSD boot drive. That really speeds up the DAW responsiveness, opening projects, etc. 
 
Then I put all Cakewalk content, sample libraries, loops, etc. on the secondary internal HDD.  If any VST plugins have large libraries, I set their path to the secondary internal HDD when I run the installer.
 
I also use external USB2 and USB3 drives for backup and archiving.
 
There are many ways that you could do this, but I have found this setup works well here.

Mobile workstation laptops usually have space for more than one drive, especially if they fit M2 spec SSDs. E.g. mine fits four, two M2 type and two regular 2.5" laptop drives. Internal drives will generally outperform external ones although the difference between an internal drive and a USB3 or Thunderbolt one may not always be apparent in everyday use. Still, it's one less port you'd need and one less item of clutter to forget. I'd recommend an internal drive for the stated use case, if you have the option.