Hi guys! I tried to enjoy Sims 4 but now I'm back with Sims 3, however my launcher won't work. I want to install a sim but she comes in a sims3pack, not a .sim file. I know I can convert her to a package file, but will she still appear in my game if I just plop her down like a mod?

You can extract the actual sim-file and place it in SavedSims, and it will show up ingame.
Was some time since i did this, so you might have to rename it to have .sim extension.

Note though anything else in the sims3pack file have to be placed in mods folder, or your sim won't have any cloths, etc that might follow with the sims3pack.

Thanks! Could you explain it more in detail? I'm sort of slow.
Also, how do I install patterns without the launcher? I found that merging them into a .package file caused CASt to load endlessly, saying something like "materializing materials".

First of all, if you don't have it already get Delphy's Sims 3 Pack Multi-Extracter or you can get S3Pack
Both will extract the content of the sims3pack, and give you a bunch of files.
Then use S3PE to see which file is the sim, if it's the sim you will see a bunch of things among them some which have the tag "SNAP" (which is the thumbnails of the sim) and some which have "SIMO" (that is the sims files), if the SIMO ones aren't there then it's not the sim-file.

Note: if S3PE throws an error message on some of them, it proberly means that the sims3pack is encrypted by EA, most likely d/l'ed from the exchange using the d/l link and not the install to game one (which despite the name actually don't install it, launcher is just used to d/l the file), there is a long discussion about that in CUSTARD's discussion thread.

When you have found out which is the actual sim-file, rename it to have .sim extension and plop it in SavedSims.
If you want it to have the name of the sim and don't know, look on the one that have the tag XML in S3PE and look for <name>*****</name>, this is the name of the sim, not sure if it need that or not, like i said was long time ago since i messed with sim-files.
The rest of the files should go in the mods folder.

About pattern's, if i remember right, there is an issue if one try use them as .package files from a .sims3pack-file, i think they sadly must be installed through the launcher to work.
Also if remember right, merging them if they are .package files is not a good idea.

And btw, did you try the "reset method", to see if launcher will work then?
If you don't know what it is, it's done this way:
Rename or move The Sims3 folder in (My) Documents, then start launcher/game and a new one will be created, which is as game kinda will see as it was just installed.

I suggest using s3pack/s3su. There will be an .XML file and a .package file that share the same name. This is the sims file. Rename it to "some name.sim" and plop it inside the SavedSims folder. Plop the other .package files in Mods/Packages (I suggest in a subfolder there). Done!