no, at the moment there is not a downgrader for 3.40, if and when there is, it will be all over the internet, just keep googling downgrader for 3.40 psp, it may take a long while so, just have patience and dont think about upgrading
oh and if you do find one, make sure others have tested it to confirm it works, some assholes like to build them and call them downgraders, but all theyll do is brick your psp