Hey guys !
Just a quick heads up : if you didn't pick up the third weapon at the end of chapter 2, just keep playing until you find the 4th one. After you have it, save your game, then reload the save and you will have the one you missed in your inventory. You will be able to pick up the next weapon again so you will have all of them.
If the game proves too hard with or without the stronger weapons, remember that you don't need to fight : running and blocking is your best friend ! Hope that helps, have a good one !