I emblemed my 3*s for tournaments originally. The original loot had a small chance to win 100 emblems, and moderate chances at 40 or 35. Considering it takes 233 emblems to fully max a 3* hero, the investment to spend emblems on 3* heroes at the time seemed trivial to me, as the return of emblems spent vs gained was definitely worth it. You could get all the emblems you spent on your 3*s (and more) within 6 months of tournaments.
I’ve placed in the top 1% every single time for the 3* tournaments. I have won 30 sometimes, most of them time I’ve only gotten 10. It makes the investment of emblems on 3*s a lot less worth it; you’ll still see a return, but it’s going to take a lot longer.
Still, I’ll keep them on, because I’ve come this far now and I don’t see the point of spending gems just to get a few emblems back.
I do agree that it takes time to form strong teams of 4*s, and in the meantime your 3*s can fill in the depth of your war bench. Emblems can definitely make them better, but I do think that choosing good ones will make more of a difference.
I’ve noticed Greymane has been getting his fair share of emblems, but I don’t really get why. He’s still a no-go for me. I haven’t had a single issue taking down defense teams with him, he’s tanky but his damage output is still pathetic. He doesn’t heal himself back much nor deals a lot of damage to my heroes, he’s a worse version of Elkanen (yes, I said that, he’s worse even considering the rarity tier). Gives me a free pass to send tiles his way and charge skills. Maybe he is better on offense? Perhaps, but I spent the emblems on Gill-Ra, a hero with a much better support skill that helps out the whole team, compared to Greymane’s self-heal which just helps himself. 
Berden, now that’s a 3* hero whose worth emblems imo 
I also spent a few on him myself.