I like both Lucas and Lamela, both have their weaknesses and frustrations but I would hate to see either leave. Lamela is certainly more productive when you factor in his assists, Lucas just doesn't produce anywhere near enough. Lamela has a very strong record of goals and assists per minute.
Now for Fraser, I can't say I've watched that much of him but my friends were excited when we were linked with him on a free. If we could get good money for Lamela which we used on a striker, defensive midfielder, or any defender then I would understand selling him and replacing him Fraser.
Edit: I picked Lamela out of the 2 due to his injury troubles, if he wasn't so injury prone then I believe he would have been one of the first names on the team sheet.