I agree with your choice to drop Graham and Tamme. I think your third drop should definitely be Plax, not Tate though. Here's why. Plax is now 33 years old. He hasn't played a snap of football since 2007. Anything he does this season will be his attempt to even get on an NFL team. He'll barely be fantasy worthy. At best he might be a bye week fill in, but honestly, I don't see him doing much at all. We have a great thread with a number of people weighing in on this exact topic in Football Leftovers if you want more than just me saying it. This thread also has some discussion about the impact of WHERE he lands and how that will affect his value. The consensus is at best he'll be a bye week fill in that could maybe get you a few points, and not much else. (found here
As for why I think you should keep Tate, here's my take. I love the kid. He's a fast deep threat, something that the Pats have lacked since the departure of Randy Moss. He's in his third year (if you believe the 3rd year WR "strategy") and anyone catching the ball from Brady is worthwhile. NE is still a pass first offense, and they've got good slot guys in Edelman and Welker, two good TEs, but they really lack a deep threat. If Tate can step in and play this role, he'll be very valuable. Worth a keeper pick and definitely a good guy to have in a dynasty league.