As for stacks for Stacks3 - Depends on your interests, goals, and needs… responsive layout, image warehousing, store, photography, writing…???
There are a lot of free one’s too that are great so a good place to start is to locate those and give them a try. (And if useful, do then give a donation as appropriate.) Some developers also have trial versions so you can try before you buy. Here’s one of many… https://stacks4stacks.com/ There are a lot of good stack developers so be sure to check them all out. Read, read, and read, the “stack” forum here to get ideas, solutions, options, etc.
Some stacks are very very useful once you figure out why you need them… it’s not always apparent. For example, I used the 2 and 3 col stacks as normal, but fount that Adaptive Grid gives much better control of breakpoints, etc. Useful stack is another one where you need it, but you may not not know it.
Some stacks you just know you need because it’s obvious they fit your niche perfectly.