They are all commonly used. Just remember how to use them respectively. Here are some examples: “洪水" usually goes with "涨(rise)", while "大水" is more used in spoken Chinese and it is often accompanied by "涨", too. They both emphasize the amout of the water. "每年夏天，我的家乡都会涨大水", "每年夏天，我的家乡都会涨洪水", that is, every summer, my hometown is hit by the flood. "水灾" emphasize the disaster the flood causes, almost the same as "洪灾". It goes with "爆发". For example, "台风季节，这里容易爆发水灾." Hope that it will be of some help to you :)

语境上，洪水跟大水是较中性的词。 水灾贬义成分居多，洪水次之。 报章媒体上，三者都很常见。 若只是单纯指水太多的话，我会用大水。 这里补充一点， 「水灾」比较强调的是它的「灾」害，而不是它的「水」多。

洪水 = 水灾 if you want to use 大水 to express 水灾， you must use "发大水" ofc u can also say 发洪水 or 发水灾. but 洪水 and 水灾 can be used as a noun, but 大水 cant. in fact, 大水 doesn't exist. u can just say 发大水 发大水 = 发洪水 = 发水灾 发大水 is less official.