Adj. + 于, where the adjective X is actually a stative verb means "more X than".
Stative verbs, which in English are normally translated as adjectives, are verbs that describe qualities, and can be modified by intensifiers as 很，太，非常, etc.
低 means "low", it is a stative verb, therefore 低于3度 means "lower than 3 degrees". If you want to keep some morphological correspondence with English, you can render is as "to drop" (= to go lower than).
As for your example sentences:
最高气温很少高于40度 here 高 (high, tall) is a stative verb, or adjective, like 低 (in fact it's its antonym), therefore this is a correct sentence, where 高于40度 means "higher than 40 degrees".
最高气温很少超过40度 I think 超过 is not the best verb here, but the sentence sounds fine to me.
最高气温很少达40度 here 达 alone doesn't sound okay; I would say 达到
最高气温很少降到40度 the word 降到 means “to fall, to drop" so this sentence is borderline nonsensical (= the highest air temperature rarely drops to 40 degrees). You can use 升到 "to rise" instead: 最高气温很少升到40度