I've saved after the mission "Big Smoke" which seems pretty useless but,I've added a little twist.
A bit of help which is:
Hid a Shotgun in Ganton.
Hid a Desert Eagle in Idlewood.
And the last one's name and location will not be mentioned just simple hints:
Heavy,Height,Heat. with 200 ammo,it's in Los Santos that's all I'm gonna say.
As far as my testing went you won't need any mods to make them appear.
They're all possible to obtain without any stats nor parking cars.
More tips?
Heavy and Heat are they weapon and Height is the location.
This is for slot 2.
Screen shots provide a small picture of the weapons plus place except for the last weapon.