Did you create an account on Eurostar and log in to it, indicating that you are a US resident? If you do that, you get access to additional cheap fares that you otherwise don't see.

I thought they'd gotten rid of one-way Leisure Select fares, but once I created an account I found they magically returned.

Also, Standard fares are quite low for US residents. I don't know the dates of your travel or the age of your 3rd family member, but you could actually go one way for as little as $225 for the three of you if one of them is under age 25, and for $270 if you are all over 25. That assumes you can go on the days and times when that fare applies.