Is leaving proraso on for five minutes instead of three anymore beneficial? Why would three minutes be optimum instead of five?

First, welcome.
Every face is different. Only your face can tell you what works best for it. Try different times; listen to your face.
And for what it's worth, I've never let lather, be it from soap or cream, marinate my face. But that's me.

But why would someone say 3 minutes is optimum? What would they mean?

I don't time how long the soap stays on. Warm shower, warm face rinse, warm brush, and face lather. Never had an issue.

Yeah

Imho unless your whiskers are literal steel wool, it will make very little difference how long you leave product on your face before shaving. But, feel free to experiment to find out what's best for you.

Doubt we'll see the OP again. Two posts in three days - gone.

Dang. I kinda wanted to know which; three minutes or five minutes.

Maybe he's busy experimenting with three versus five minutes.

Depends on the time zone.

If he's on the west side of town in Mexico Beach, FL, he can lather up, go across town, and say he's let that lather sit for an hour!

Why would you want LYE on your face longer than it's necessary to shave?

Next thing we know, you will tell us not to wash behind our ears. Where will the madness end?

Well, washing behind the ears does tend to remove the scent of the veg. At least partially.

I once was so enamored with the scent of Tabula Rasa Steam Punk, I left the lather on my face and watched some TV. Bad idea...just saying.