Definitely childbirth ;) but on the serious note you may find being more tired and everyday activities will be much harder to do.
The renewed energy that you found in the second trimester will be gone! I've experienced a lot more Braxton hicks as well
Expect to move way slower and more leg cramps along with being a bit more out of breath and heartburn lol braxton Hicks are worst than before for me too plus for some reason i urinate so much more lately