If you have the time to run 6-7 days a week I would think you would have a better chance of going sub 3 with all easy runs then 1 long run with MP. 3 hours is a speedy marathon pace even if many on here can do it. And the more miles you have on your legs the less chance you will have of tiring out running at 6:50 pace. And if you do all but 1-2 runs easy, and I mean easy, you can still hit a tempo/vo2 workout during the week to pair with your long run with MP. Say you are able to run 7 days a week…
Monday: 5-6 miles easy
Tuesday: 5-6 miles easy
Wednesday: 10 total with tempo/VO2 intervals
Thursday: 4-6 miles EASY
Friday: 6-8 miles easy
Saturday: 6-8 miles easy
Sunday: 16-22 miles with 30 minutes MP adding 5 each week
That is a total of 53+ miles with most being easy which means you should be able to ramp up to that mileage if you take care of yourself and run your easy runs easy. I am not a run only guy and have found it is insanely easier to find time to run than juggling all 3. 10 hours a week running is like 65-80 miles a week depending on pace and 10 hours a week seems to be light work for many triathletes. So if you keep yourself in check you have time to get to this mileage and stay there and have a better chance at sub 3 than if you only ran 3 days a week. And this is for most people, of course there are outliers who can run little mileage and still run a fast marathon.