The #3 receiver is at best the #5 receiver target. Perhaps #6 is more reasonable, after whichever running back is in the game at the time.
As DI has indicated, perhaps we will have a greater role for the #3 and #4 receivers in 2012, using 7 targets in a game (4 WR, 2 TE and 1 RB). In 2011, we clearly overused Welker and Gronkowski. Our top 4 receivers accounted for 85% of the receptions, with the running backs accounting for 9% (the rb number seems reasonable).
WIDE RECEIVERS OTHER THAN THE #1 and #2 IN 2011
They accounted for a total of 23 catches all year, 1 1/2 catches per game. We should be able to spread the ball out better. Welker doesn't have that many 100 receptions season left in him.
3 ROSTER SPOTS
Obviously, the best 53 make the squad, and an extra wideout is always possible. That being said, you arithmetic is spot on. We need THREE more receivers, with one being inactive in almost any game. At least two of these are expected to play special teams. At minimum, we need one to be a returner.
I agree that there is a lot of competition for these three roster spots. As you say, we have Branch, Stallworth, Johnson, Gonzalez, Edelman, Underwood and perhaps a draftee.
We should be real. There should be no more than 5 WR roster spots. We have four fine receivers targets (2 WR and 2 TE's) and the RB's should get some receptions.
A SIDE NOTE ON THE "LAST" FEW ROSTER SPOTS
Some STer are locks. We just don't recognize the importance Belichick puts on special teamers. A spot or two can be used for developmental players who are expected to be inactive this year, but are two valuable to risk trying to place on the Practice Squad.
And last couple of spots simply go to the player who is most valuable, which can change from game to game.
I won't say it is useless to project the positions played by the last 3 roster spots, but that is almost the case.
ONE BOTTOM LINE
The #5 WR position is not very important. We need four healthy wide receivers, that's all. The four must include a returner.