If you try to put 3 people (1 MVPG + 2 Guests) into Premium through the website, it's going to charge you the extra fare for all 3 seats - including for the MVPG. I was looking at this exact scenario not too long ago.

If you've already booked all 3 together on the same PNR, you might be able to call an agent and ask if you can move all 3 seats to premium, but to waive the premium fee for yourself and your one guest that should be eligible for free premium, but only pay for the 3rd. Your response may likely run the gamut anywhere from them obliging your request as stated, or maybe they'll give you all 3 for free, or maybe they'll tell you they would have to charge for all 3. YMMV