https://www.gsmarena.com/motorola_moto_g_(3rd_gen)-7247.php
https://gadgets.ndtv.com/motorola-moto-g-gen-3-2817
Any Android phone with only 8GB of storage is going to be limited. That's simply not enough for most people to use as a daily usage phone in real world situations. You either have to set your expectations to match its limitations, or spend a lot of time on a constant and frequent basis clearing up a few MBs of free space.
If it's still running Lollipop you can only use that microSD card as a typical, portable card. If it's been upgraded to Marshmallow, and if Motorola enabled this, you have the option to format the card as either portable or internal (a.k.a adoptable). If the former, just putting the card in your phone doesn't just free up storage space, you need to install a file manage app and move files like photos, videos, music, etc. to the card. If the latter, the card essentially merges with your phone's internal storage -- you don't directly interact with the card any longer as it's more or less now an additional component of your phone. Also, your G3 only supports up to 32GB microSD cards so if you put in a larger capacity card it may or may not be a stable memory media.