I stand corrected. I did know quite a few Danes who would use English to converse with their Northern neighbours. Either they were outliers, or I assumed the differences between the three big norse languages were roughly equal.
So, I am an American who lived in Sweden for roughly 4 years, long enough to consider myself mostly fluent (reading novels, talking about relatively high level political science questions in Swedish, etc.) and only agree partially with the comment above yours. Norwegians and Swedes would speak in their native tongues regularly but a lot of the time, especially if either person was from a region with a distinct accent (Skåne, i'm looking at you), they would transition to English.
As for Danes, that is a different boat. Their written language is extremely similar to Swedish but their pronunciation is far more guttural and hard to understand for many Swedes. I went to Copenhagen with a bunch of Swedes and they spoke almost exclusively English to the Danes.
"Scandinavian" languages, please. Or even mainland Scandinavian, to distinguish from the (not very mutually intelligible) insular Scandinavian in the Faroes and on Iceland.
> I did know quite a few Danes
That last word is the explanation. :-) More below.
> I assumed the differences between the three big norse languages were roughly equal.
Swedes and Norwegians have little trouble talking with one another in their own languages. Norwegian written language looks more similar to Danish (indeed, bokmål is highly similar) in that a Dane would have no trouble reading Norwegian or vice-versa. Written Swedish differs in more than just the last three letters of the alphabet.
Spoken Danish is a whole other animal. It works out when people try to speak as standardly as possible. Norwegians and Swedes (except some from the very south of Sweden) will struggle with ordinary spoken Danish, and are even likely to turn on subtitling when watching Danish TV shows (likewise there will be Danish subtitling of some Swedish shows).
In some technical settings you'll see more use of English among Scandinavians because there is a non-commonality of terms, and of course it is simply more polite to talk in English when talking someone whose only common language (or indeed only language) is English, and it is especially Danish to do so in order to avoid excluding anyone.