I'm not finding an obviously safe place for them. The pocket that is in the flap of the top compartment (for housing the controller) barely seems long enough and likely to induce bending in the props? The side pockets are too short.
It seems an obvious oversight actually. It's not a bad quality backpack overall, but I'm shocked. I mean, the props can be stuffed in to the controller compartment, but at some risk, imo. A long padded pocket should have been supplied somewhere for this purpose. Be aware of this issue if you are thinking of buying this pack. There is no greatly sensible place to put the propellers!
I bought the manuacturer's KATA version of the "Official DJI" bag, the Bumblebee 222L which looks identical but may be bigger. There are 2 side pockets and the props fit nicely in the taller of them. I'm assuming that you've not got these otherwise you wouldn't be asking. Manfrotto now makes the bags and if the DJI ones don't have these pouches then it's a big oversight. Even the Manfrotto version is at least half the price, I paid £130 (that's about $100,000 ;-) ) and it's a beauty. If you can still return yours I'd recommend returning it and getting one.
There was one discussion thread that listed a dozen or more backpacks for the P3, but having spent 20 minutes looking I can't find it. Can anyone give me a pointer, please?