1) 'Quote more than 1 post in my replies?
there's a plus sign beside the quote button, click on that for each post you want to quote and than hit quote on the last one you want to quote, it will take you to the advanced screen with the quotes already there
2) Update my original posts?
do you mean edit your threads? just use the edit button
3) Insert another thread into a new/different thread?
You can't merge threads, only a mod can do that. You can copy and paste links from your previous thread into the new one (though this is mostly done for journals when people want to link to their previous journey)