I'm a native of Portland, live north of Boston, and commute to work in Providence. I'm quite familiar with that entire stretch of highway.

The worst stretch of traffic during morning rush hour on I-95 is from a couple of miles before the I-93 interchange in Reading to Winter St. in Waltham, but summer traffic on Friday mornings is usually much ligher than the other days and it shouldn't be a problem even during morning rush hour as long as it isn't raining. Once past Winter St. its clear sailing to Providence unless there are work crews doing guardrail work or if they're working on the bridges that day at exit 9 in Foxboro or the I-95/I-495 interchange in Marshfield.

I-495 has its share of traffic problems as well nowadays even without taking the current construction into account so I wouldn't automatically consider it to be a superior option.

3:15 should be enough for you to get from whichever entrance you're going to use to get onto the Maine Turnpike to I-95 in Providence.

You don't say anything about the drive back but I would hope you're not looking to leave in the afternoon to drive back to Portland. Have dinner in Providence and leave sometime after about 7PM or so.