Can one recommendation letter from professor and two from internships help in MS applications? The university applications for MS in Computer Science requires 3 recommendation letters. I am confused between choosing my professors or industry mentors. 

Potential people:


Assistant professor
Mentor at a remote research related project from a prestigious university
CEO of an internet based company
Other professors/ head of department


I have a professor under whom I did several important research projects and have, also, taken his classes and labs. He is very pleased with my performance and he eager to provide a recommendation letter. He is an assistant professor at my university.

In case of other professors, I have never worked on any research projects so, the recommendation letter will be average in my opinion, as they will talk about classes and labs I took under them. This, also, applies for the head of the department. For this reason, I do not prefer to take one from them.

I did two remote internships. One person is from a very prestigious university. I am not sure as he prefers to keep the letter confidential. I did some research related work under him and basically, managed several teams.

Another person is the CEO of a company and is expected to provide a strong letter as he is pleased with my performance.

I am confused regarding whom should I approach for the letter? As this will be an important deciding factor, kindly share your opinions.
As a general rule, I would take recommendations from people who worked with me directly. Then, I would look for people who were pleased by my performance. Third, I would take the recommendations most relevant to the position I'm applying for. 

It matters to some extent how famous is the person recommending you, but I think the graduate committee would be more interested in a letter coming from someone you worked with such as the assistant professor, the professor and CEO you did internships for. That letter is more likely to be more factual and tell the committee what you have actually done while working with those people. I would be happy to see in a student's application letter that they actually managed work teams. 

In your case, you don't have to ask people you only took classes with. I remember when I had to write recommendations for students who did well in my class. I still didn't know what to say beyond what I knew from their course work. I think they might have deserved better than that, but I couldn't offer more.