Memory is a beast to get working as desired on threadripper, hopefully the newer 2950x is will be a bit more forgiving.
looking at the QVL list should aid you, in finding some decent kit, but at 128gb getting 3200 or 3000 will be tough .
http://download.gigabyte.eu/FileList/Memory/mb_memory_x399-designare-ex_250W.pdf
that said, with the appropriate tools, patience and luck you may be able to tune a good set of kit to the desired frequency
ryzen dram calculator:
https://www.overclock.net/forum/13-amd-general/1640919-new-dram-calculator-ryzena-1-4-1-overclocking-dram-am4.html
ryzen timing checker:
Thaiphoon burner:
http://www.softnology.biz/news.html
these tools will get well on your way for getting the memory to work at you desired tuning .
with that out the way all thats left is getting the memory …
easier said than done i know…
for samsung b die there is a tool that may help
https://benzhaomin.github.io/bdiefinder/
reading through that thread on overclock.net forum should be an aid also but time consuming. regardless it is well worth it for the amount of experience and knowledge contained there.
also there is thread here in this forum with i beleive wendalls 128gb setup and tests on threadripper … ah here it is ,
so hopefully armed with this you can pick out a good kit.
personally i recommend lowest latency you can find. as it seems lower latency yeilds better results but i have only tested this at 32 gb and not 128 . good luck !!