So the best practice is not to let each node fill more than 50% of the disk. That's because (Size Tiered) compaction can use double the space of the SSTable files that it's running on. Therefore compaction running on your 850GB file will temporarily and abruptly use another 850GB of disk (during the process).
Also makes sense to increase nodes by a factor of your RF. So if you have 3 nodes, I'd recommend going to 6 for even data distribution.
I'd also recommend upgrading to the latest 4.0 version. Just me, but I wouldn't run a beta version in prod.
I could put bigger disks in the machines. Is that a good idea?
Sure, that would do it, too. However, the best, recommended practice is not to go too much more than 2TB per Cassandra node. I do know of folks who have gone beyond that, and run just fine. But anything that requires streaming data between nodes (ex: repair, adding or remove nodes) takes a looooooong time.