Tracing up is easier than tracing down or across in my experience. The ancestry tools allow you to go backwards much easier.
If you have a match who has a tree you can find the common link and then trace back down to them from their tree using their paper trail.
The problem comes with people who are still alive - they are automatically set to private if there is no death date and they are born in the last 100 years.
So even if an sibling or a cousin posted a tree with their uncle/brother (the OPs husband) on, you'd still be unable to see them.
You'd then have to use the search tools to find siblings. Which can be done but isn't necessarily that easy. The ancestry search tools can be clumsy to say the least.
Most people who do a DNA test only use the month's free trial and a great number down post a linked family tree to their DNA profile. Even if they do they often are set to private, or only have very limited information (or no information) about dates and locations.
This means that you actually have to get lucky with the match you get OR make contact with the match and hope that they reply and give your further details about living people. Thats why I think there's likely been contact first.
The only other decent explaination is the three siblings have been in contact and compared their mother's accounts and names - but this might not mean its easy to actually find the father's location unless they have a more unusual name.