Someone's gonna need an extra third baseman and he is still in his late 20s so he's bound to catch on somewhere. He is only a couple years off of a decent season in Colorado (.810 OPS/105 OPS+) and he was a top-10 draft pick so there is a chance he could be a late bloomer. That being said, 2013 wasn't that great of a year as a bounced around to three different teams and the numbers indicate that he's struggled defensively. His most similar player through age 27 was Hector Luna. After 27, he had 97 plate appearances spread out over three different seasons. For Nelson, this year is likely the turning point between whether any of that promise that got him drafted early turns out or if he has lots of minor league games in his near future.
Scott may not be getting older and he may not be that durable and he may not be that good with the glove but he still has enough pop in his bat to help somewhere as either a part-time DH/pinch-hitter. His batting average/obp may indicate that he would be a bad option but he's hit for enough power to make an pretty much a league average hitter. His most similar player through age 35 is Dale Long had one decent season left in him. Scott could be a sneaky good one-year contract signing if he ends up on a team that utilizes him correctly.
For much of his career, Gonzales was a good fielding, poor hitting shortstop. He's played 65 games over the last couple years for the Brewers. In 2012, he was actually off to a decent start before hurting himself stealing a base. In 2013, he was part of an ill-advised first baseman by committee that was an utter and complete disaster (18 OPS+ while somehow getting 16 starts at first). So basically, he's washed up. His most similar batter through age 36 (Damian Easley) had a couple decent seasons as a part time player afterwords. This may be one of the friendlier projections based on current free agents and similar players but from what I saw from him last year, I would be very surprised if whoever signs him got similar results.
Hafner is one of the career what ifs. What if his career hadn't been marred by injury. If that was the case, he likely wouldn't be in a posting about free agents. We would be more likely to be debating his merits as a potential hall of famer (I forgot how good he was in his 2004 to 2006 peak). He hasn't had more than 462 plate appearance in the last six years but has hit pretty respectably in the limited samples. He brings nothing to the table defensively so he's likely to be constrained to the American Legaue and hope to get a DH gig somewhere. His most similar player through age 36 (Dick Stuart) was done after that season. I think Hafner has a bit more of an optimstic projection for at least the next year or two and I could see him being a good acquisition for an American League team.