1. Danish
2. Norwegian (especially Bokmål)
3. Swedish
These languages are part of the North Germanic language family and have evolved from a common ancestor. This shared linguistic history has led to a significant number of cognates (words with similar origins) and similar grammatical structures. While there are differences in pronunciation, vocabulary, and grammar, speakers of these languages can often understand each other to a reasonable degree, especially in written form.
It's worth noting that:
* Norwegian has two main written forms, Bokmål and Nynorsk. Bokmål is closer to Danish and Swedish in terms of intelligibility, while Nynorsk is slightly less so.
* Icelandic, while also a North Germanic language, is significantly less mutually intelligible with the other three, due to its more conservative development.
Therefore, while a native speaker of one of these languages may not be able to fully understand every word spoken by a speaker of another, they will generally be able to follow the main points of a conversation and get the general gist of what is being said.