Hiya, I would start by suggesting to use the 
search function of the forum as there are a large number of threads on “Which are the best 3* heroes” and “Duplicate 3* Heroes”
Yes most of them pre-date the S3 heroes but they are still pretty useful for the most part.
Personally I have kept 1 of every 3* hero and have duplicates of the following:
- Brienne - costume & non-costume for events/ tournaments
- Rudolph - main man in event teams (actually have 4x of him)
- Berden - alternate team for events
- Hissan - use in tandum with dual berden for a mono green event team
- Gato - Alternate team for events with Vodnik, Valen & Gunnar Costume
- Pixie - Mana control…
- Bat - Plan to level both & use as another event team
Edit: for S3 heroes I have (and will) keep duplicates of the following. Rest are just single copies.
- Ei-Dunn (fun to just wipe an enemy squad)
- Bjorn (better than Balthazar imo)
- Nodri (new event beast)
And thats it… every other hero I only have singles of