Depends where you're at. I think there's no reason to max 3 stars unless you will use them for Variant 4, or current story content, or if you're a content creator who wants a new arena champ
If you’re intending on maxing out the milestones consistently then it’s worth it. I’d say you’re probably better off just using the free T2B from weekly objectives and slowly feeding them into your champs though.
In my eyes, doing arena is a lot less important than it used to be since 4 stars (and 5 stars depending on if you're UC/Cav or not) are much more widely available. The only real use I see in it is Unit farming.
I guess it depends on your reasoning. I rank up a lot of my lower star champs to hit milestones cheaply and it has turned into something I have made a goal of. I have 130 2* champs all max rank and 150 of my 173 of my 3* champs at max. Eventually I hope to start moving on to 4*s down the road as well but resources are key. Advancing in the game is important, this is just a side project and fun for me.
Worth to rank up for level up event alone, unlike 4 stars, it does not use any of the t1a, t4b that you need for ranking up 5 stars. 3 star arena is only worth to do if you have lots of 3 star arena boost
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Beyond that, you're most likely only going to use them for the occasional synergy, so they won't achieve much.