If you’re getting Phone (3) at a good discount, I’d say go for it. At full price, it’s harder to justify.
Phone (3) already cut quite a few corners while being priced high, which is why it didn’t sell that well initially. A lot of people picked it up later when Nothing started pushing discounts, loyalty bonuses, etc.
As for waiting for Phone (4), it could honestly go either way. It might come with better specs but be priced higher, or they bring a new processor and some performance improvements while cutting corners again in areas like RAM type, storage, or cameras and try to price it better than Phone (3).
With the recent RAM price bump, I don’t expect some massive feature jump next gen anyway. So, if the deal is good, buy now. If not, waiting makes sense.