The post has always had this little side to side play when you grab the saddle and waggle it probably around 2-3mm I guess

Is this normal?

I dont want it to get progressively worse – you can feel it when you push down on the saddle as well

Warranty job or not?

Post only about 3 months old

2-3 mm rotational play at the front of the saddle is normal

Side to side play normal. Front / back not.
When you press dowm the saddle are you sure its not the saddle flexing or loose rails?

I just got a warranty replacement and it does this, exactly as you describe. I’m assuming they wouldn’t have sent me a faulty item as a replacement for the original faulty item, so hoping as iainc said, it’s normal.

a small amount of rotational play is normal. If they put any tighter locating pins in, the post would be slow/stiff to go up and down. In reality, 2-3mm at the nose cant be felt when you’re riding anyway…. unless you ride like a spakker of course 😆

In addition, following on from a post, (above), on an old post, you may eventually get a few mm of movement from front-to-back… this is not a warranty, its a sign that you’re hammering the bushes that allow the post to run up an down smoothly and it may be time to get the bushes replaced at the seal head and the collar/top cap assembly. That’s just normal wear and tear really.
Replacement of post keys (which are a sacrificial/service item anyway) and bushes would be considered as a service item.

If the movement can be felt when riding, I would be mindful to check the saddle rail clamps are torqued correctly.