如何将所有子列表元素组合到一个列表中

I have a list (of length 3) which is made up of sublists (each of differing length - 2, 2, 3). I would like to store all of this as one big list (e.g., no sublists - just one list of length 7). I understand how to do it manually, but is there a function or command I can use? I have a list (of length 3) which is made up of