Say you choose door 1, and supposing the host gives you the opportunity to keep the contents of door 1, or to swap and keep the contents of BOTH other doors. Obviously you’d swap, because that gives you a 2/3 chance of winning. The host then says I’ll save you the bother of opening both doors, I’ll open one of the doors for you.
The only difference in the actual game is that the host opens one of the doors before asking if you want to swap instead of after, but that’s irrelevant.