I'm not the least bit worried about 3* OLinemen or DLinemen. It is easier to project on the DLine, so naturally a higher star rating would be nicer. In the end, these kids have to have time to physically mature to make a real difference. There are always exceptions (and Cowart may be one), but I'd take 3* lines and 4/5* skill players all day. I would say that a caveat is a speed rushing specialist.
Here is an example, in 2007 we signed John Brown (DT), Torrey Davis (DT), James Wilson (OL) and Carlos Dunlap (DE). All were considered either 5* or very high 4* depending on the service. Brown and Davis (save 1 play) were busts. Wilson had basically 1 good year, and Dunlap was sporadic at best until his final year. That same year, Urban pushed Linvel Joseph out of the class to make room. Joseph was a 3* kid from the Gainesville area who now has a SuperBowl ring. If you're willing to do the research, you can find a bunch of these stories. The lines are very hard to project, especially OLine.