You can upload PS3 Gamesave to PSN Cloud, but your PS4 will not show it or other else PS3 saves to you.when you want to download it. I dont tried it with "PSNow" but even then i would say - it will not work. I have all PS3 Saves in my cloud too, but they are not listed when i check them on my PS4. Some Games use "Online-Server-Save" - they save the progress of onlinegames on their Servers. When you use a game with this feature, you can transfer the save from there to all your consoles. Otherwise there is no way :-(