Which three should I keep?
1. A.J. Green
2. Brandon Marshall
3. Matt Forte
4. Michael Turner
5. Matt Ryan
6. Vernon Davis
It depends on how many teams in your league, how its scored, etc. Lets just talk about a typical scoring league of 10 or 12 teams. Here is the 3 step process I use to decide on keepers:
1. Decide how you would rank each of the players you are considering.
2. Calculate approximate range of picks
3. determine whether that player falls in the range, is drastically over, or drastically under.
1. Determine how you would rank your players you are considering. This can be done using other guides as a starting point or create your own. I would suggest starting with another guide, or see the average draft position chart some FFB leagues provide. That's a good place to start.
2. Calculate the approximate range of picks. You have said you can pick up to 3 keepers. If your league has 10 teams, you need to find 3 players in the top 30(3 rounds x 10 picks per round). If your league has 12 teams, you need to find 3 players in the top 36 (3 x 12).
3. How many of the players you have listed would be top 30 or top 36?
-Matt Forte: I have him as a first rounder so a top 10 player. Top 10 is drastically better than top 30/36. So i'd keep him.
-Brandon Marshall: I have him in the mid 20s. We were looking for 3 in the top 30/36. Mid 20s is within range. I would keep him.
-AJ Green: right around a top 30 player. Here is where it gets tougher. Its a tossup. He is within range, but there may be better options available such as Julio Jones, DeMarco Murray, or Jordy nelson may be available. Personally, I'd keep him.
-Turner: I have him around 35. He is iffy, but I wouldn't take him. If I could only keep 3, I would take Green over Turner. Green has the potential to be an elite WR, Turner does not. There are also probably other good RB options available here(maybe McFadden or Jamal Charles for example).
-Matty Ice: I have him in the 60s/70s. That is well outside the top 30/36 paramters. Outside of the big time QBs, there is a big group which is not that different from each other stats wise. Grab one of them and save your valuable keeper spots.
-Vernon Davis: Pass big time. SF does not throw enough to justify making him a keeper. I have him around the 70s. He isn't Gronk or Jimmy Graham(the only 2 TEs I would make a keeper). You can probably redraft him in the 5th or 6th round and still be happy.
That's how I would do it. Best of luck.