I’m thinking about getting a Samsung 128GB microsd as they’re cheap in Amazon and I assume not fake/shanzai. But can the Rpi3b+ use this? Also it is speed 10 and U3 rated because there’s no longer any speed 3 for sale.
According to eLinux these cards with the respective part numbers (last column) should work fine.
Samsung EVO microSDXC 128 10 MB-MP128GA/AM Samsung EVO PLUS microSDXC 128 10 MB-MC128DA/AMZ Samsung PRO+ microSDXC 128 10 UHS-I U3 MB-MD128DA/AM
Besides these a few others, e.g. SanDisk are listed as working.
If you have a Linux device up running, you can use this F3 - an alternative to h2testw utility to check the integrity of your microSD and/or USB storage against the real size of its storage. It is available on debian Stretch. The software also prints out the R/W speed on-demand when checking/testing the storage. This way, you will not only know its integrity, but also will know its R/W performance throughputs. I use it all the time when I acquire any new (micro)SD cards and/or any USB memory sticks.