VNAV path is not based on a 3º descent, but if you have no altitude restrictions during descend and light winds, it will be something very close to a 3º descent. For example you`re cruising at 350 and the next point to be is the LOC capture point to be at 3000 feet and at 180 kts. You enter that into the FMC and VNAV path will put there at that altitude and speed. But since it`s path we`re talking about, it does garantee that you will be there at 3000 feet, but doesn`t garantee the speed! It will be slaved to altitude. If you wanna comply with the speed, you should use VNAV speed.
If you feed the FMC with expected winds and speed you wanna fly, I usually use 320 Kts for descend and 250 bellow FL100 and 180 to be at the IAF, it always work pretty well!
Cheers!