Hi.
TWRP was easy to find and update.
About the firmware I am unsure which one to use. There are severeal threads, links and versions, for example:
https://forum.xda-developers.com/oneplus-3/development/firmware-updates-oos-3-1-2-3-5-2-t3465312
Others tell about "this is OP 3 only, not 3T..."...