I've personally use 2 ssd's and 1hdd.
I've use GameSave Manager's Steam Spreader to move older games from ssd 1 to ssd 2.
The hdd has always just been for documents, downloads, favorites, music. pictures, saved games, searches and videos.
OS, programs and newer games on ssd 1.
Older games on ssd 2 - that are o.k. with being moved from C; using symbolic links.
I recently upgraded my 2 ssd's from 1 Samsung 850 EVO 500GB and 1 Intel 530 480GB to 2 Samsung 850 EVO 1TB ssd's (one m.2 ssd and 1 sata ssd).
I needed more space on both for more games, etc.
I'd definitely go with atleast a 1TB Samsung 850 EVO or 950/960 EVO.
It's a matter of what you can afford and how much space you think you'll need down the road.
For me the 2 1TB ssd's made sense and should be good on space for 2-3 more years.
Most games are getting bigger, so plan ahead for the space you'll need.
P.S. Never put your games on a hdd at this point in time. They run better on a ssd.
Also stick with Samsung for ssd's. Samsung Magician's Rapid Mode kick's ass performance wise.