How do I used the Sims 3 expansion packs I've installed on my PC from Origin online?

did you download origin, to download your games  ?  if not download https://www.origin.com/en-us/download use the same email and password for the sims 3 account as your origin account and the sims 3 are tied together https://help.ea.com/article/how-do-i-download-my-new-game-on-origin  then download your games from the mygames tab if the games do not show up right away log out of origin and log back in

just in case you get aDLC alert while downloading do this to be able to install the download version without reinstalling despite that message, do not click on the download button in origin click on the “i” (information) button  this will open another window that also has a download button click on that to install