If you don't want to use flex-box (even though you will have more control, I really can't get my head around it at all)....
Set the dynamic list on the desktop to be 1 column, so it stretches all the way across. Then set the dynamic item to be 33.3% and push to the left. Don't set a height, use a div within the dynamic item to set the height.
On tablet, set the dynamic item to a width of 50%
On mobile set item to have 100%
Hope that makes sense, I'm happy to share a project if you need to see what I mean. I've just used this on a live website with great effect.