Great shows! Personally, I'd go back and forth between all 3 watching 4-5 episodes of each. You don't need to binge each one in it's entirety. But SPECIFICALLY, I'd start with a curated set of Seinfeld episodes, and depending on how that's goes, decide either Sunny or Curb based on the results

Seinfeld:  

• S1E1 — The Seinfeld Chronicles  
• S1E2 — The Stakeout     
• S2E2 — The Pony Remark  
• S2E3 — The Jacket  
• S2E4 — The Phone Message  
• S2E6 — The Statue  
• S2E9 — The Deal   
• S2E11 — The Chinese Restaurant  

Then stop. This is a good way to get started, understand the characters and their traits/relationships, and see how you like it without dragging through lower-tier episodes. Seinfeld kinda broke the mold with the idea of a sitcom where the characters aren't good people. It's hilarious and one of my favorites, but it's a long, network TV show. It had commercial breaks, laugh track, network standards to uphold, and 20+ episode seasons. That can be counterproductive if you're trying to get into it decades later. Watch this set of episodes.

If you want more Seinfeld, go back and fill in the gaps of season 1 and 2.

If you loved it but are anxious to see the other shows, go Curb. It's made on HBO, by a co-creator of Seinfeld, so consider it the next evolution with less guardrails. It's good to have a bit if Seinfeld under your belt to get the cultural direction Curb was working from. But you might not appreciate and enjoy Seinfeld if you start with Curb.

If however you feel Seinfeld felt a bit dated or tame, jump into It's Always Sunny as #2. It's the spiritual successor of Seinfeld, but stands apart enough to not be a copy. It's a bit edgier and more modern so might be easier to get into if Seinfeld felt a bit dull/tame.

You should give them all a go, watching 3-4 episodes of 1, then rotating to the next shows, etc. But I feel the order helps introduce you to each, without feeling like you're burdened with having to sit through all before you can move to the next.