The Raiders start their three-day mini-camp later today, and so far in the first three weeks of offseason practices, all the receivers have looked pretty good. “They’re a year better and a year more comfortable,” offensive coordinator Greg Olson said.

Olson, who told the Chronicle last week that rookie quarterback Derek Carr is even better than the Raiders thought, said that Rod Streater has stood out and that newcomer Greg Little has been a nice surprise.

“Rod Streater has gotten better every week that he’s been here, just works hard and has gotten a little bigger as well,” Olson said. “Greg Little has done a really nice job. I guess there were some questions about his work ethic, but I like the way he has come in and really attacked it. He’s been very consistent catching the ball every day.”

Little, 25, was cut by the Browns after a 41-catch, 465-yard season where he again had a problem with drops.

While the Raiders are rotating receivers right now, and Dennis Allen has shrugged off any talk of depth charts, observers say that James Jones, Streater and Andre Holmes are ahead of Denarius Moore, who started last year.

Holmes came on at the end of last season and the coaches really like his height (6-foot-4) and his ability to high-point catches. His 366 yards over the final five games ranked 15th among all NFL receivers.

Moore, meanwhile, has flashed big-play potential but caught only 33 passes for 618 yards in 13 games last season, his third. He had a couple of loud drops at last Monday’s practice (the one reporters were allowed to watch). Last year, both coach Dennis Allen and Olson openly questioned Moore’s focus and attention to details. I asked Allen last week if that had gotten better.

“I think we’re still working through that,” Allen said. “He’s not unlike a lot of guys we have on the football team. He’s still got a lot of improvement that he’s got to make. I’ve seen some strides and hopefully he’ll continue to make those strides. …

“With all these guys attention to detail is where we’ve got to work to continue to improve, and that’s with all these guys. Because the margin for error in the NFL is very minute, very small, so the ability to be precise in everything you do is critical to having success.”

Given that’s five receivers we’ve mentioned already, it appears that its going to be tough for Brice Butler, Juron Criner or any other of the receivers here to make the roster this season.