They aren't one third of Congress. You might be thinking of them being one third of the Biden cabinet. There are many sources for the Congress one such as: https://www.jta.org/2021/01/11/politics/here-are-the-36-jewish-members-of-congress

For Biden's cabinet, Eric Lander's resignation brought it down 7/25 Jewish. It was 8/25 for 32% so approximately a third. You can just do early life checks via Wikipedia or look at Jewish press such as this. That articles is a bit out of date as they tend to be since most of them covered his incoming cabinet. There are other articles you'd have to dig up for the departing and newer members.

I'd like to highlight the biggest ones: Federal Reserve, DHS, Secretary of State, Director of National Intelligence, and Attorney General. That's a crazy amount of power in those five positions.