What are the Gen3 pokemon worth keeping?

Delibird.

Wait, that's gen 2, brought out along with gen 3 #nianticlogic

That plusle pokemon is great. Appears everywhere and evolves into nothing. Power them all up.

Till today, with half of gen 3 (aproximately) released, I don't see any potential pokemon worth powering up for real use (my perspective). There might be some later on.

Till now the pkmn that might be considered are Blaziken or Hariyama. You can check for yourself here (https://www.youtube.com/watch?v=OmZioRepteM)
Mabye Wailord for defending, but Raikou will counter hard.
Also Sceptile, still not sure, but i think Exeggutor is still better.

I think that's all I've thought for now

Gardevoir is very good. Sceptile also not bad.

Walrein will occupy a similar spot as Lapras. Which is to say no man's land. I guess it has somewhat better attack so you could use as an ice attacker, and has waterfall so it could be an OK defender. So maybe Walrein is just one more nail in Lapras' coffin.

- Walrein serves on gym defense similar to Lapras, trading a small amount of its bulk for higher attack and better Water-type options.

- Milotic is virtually identical to Vaporeon in terms of tankiness, but has much better moves for gym defense and can also attack outside of its type. I would argue it's the best water-type gym defender now, since Gyarados has that double weakness.

- Gardevoir is prized for its double resistances to both Dragon and Fighting while also losing its weakness to Dark (All which currently rule the meta). It's by far the best Psychic-type on gym defense. It makes a fantastic addition to any gym (especially for EX-Raids) because it complements Blissey, Tyranitar, and other common gym sights so well, while having an amazingly good gym defense moveset (Con/Dazzling Gleam or Con/Shadow Ball). Strong, semi-fast two bar moves with good type coverage paired with a STAB move with high power and low speed. Also great on offense and the best Fairy-type attacker, using double resistance, effectiveness, and STAB to destroy Fighting-type counters. Despite the relatively low max CP, I think Gardevoir is the real winner here.

- Slaking and Wailord are completely overrated to me. The former is plagued by predictability and CP decay, while the latter benefits from it. The latter, however, has very low base stats (don't overestimate its low defense), while the former benefits from high base stats.

The best addition, IMO, Gardevoir who is the best Dragon slayer (it not as fragile as Jynx, has good attack stat and resists Dragon type moves). Also try to get more Ralts candies bc in Gen IV there's an alternative Kirlia evolution which might be as good as Gardevoir.
others are pretty specific. Breloom and Blaziken should be great against specific matchups but in general Machamp is still essential. Sceptile is quite fast and has advantage over Eggs but it's not like it outshines the Palm tree. Swampert seems to be not as painfully slow as Rhydon due to Mud Shot. and Hariyama is a budget more bulky Machamp
Walrein and Milotic are decent gym defenders but they are not surpassing Blissey, of course.
Also the legendaries: Groundon, Kyogre and Rayquaza obviously. They are probably gonna be the best attackers of their respective types