I have 3 years of work experience in cloud computing and virtualization and am intending to illuminate this in my SOP. With what degree of credibility will the above mentioned work experience be accepted by admissions committees if I do not have any letter from my workplace at all? Also, since it is difficult for me to get a recommendation from my immediate manager at my workplace, if at all I do get one from my company, it may be from a colleague who was my mentor (no managerial role). How good will such a recommendation be? I already have two recommendations from my college professors, each of which highlight one of my two research publications. It is for the third recommendation that I have the choice between this letter from my mentor at my workplace and from an assistant professor at my college who had taught me in a number of courses and labs. Which recommendation should I go forward with? Please advise on these matters.
Unless the MS is very applied, I'd think that an additional academic recommendation would be better. That is especially true if they can speak to different aspects of your work.
But a letter from a mentor should, in general, be fine. It might even be better than one from a manager as the person likely has more knowledge of you and your contributions. The letter from a manager is likely to be a bit pro forma. The recommender should be clear about the relationship (i.e. mentor, not manager).
Speaking from UK institution experience, candidates who have "real world" experience in roles relevant to their Masters course tend to be favoured much more than students who are applying directly from undergradute, as it shows that you've put thought and consideration into your choice of course and so are more unlikely to drop out.
I would argue that your mentor, and so someone who likely interacts with you much more often than your manager probably does, would likely count more towards your recommendation anyway, as it is more bespoke. I would advise to go with the current choice of 2x academic and 1x professional, otherwise it may come across as a little peculiar that you have this 3 year work experience but no one to recommend you from that role/institution.
I review applications to our master's program, and it would seem strange to us if someone with years of work experience did not include a letter from their workplace. I don't think it would matter much to us whether it came from their mentor or their manager. It's useful to us to know that someone can show up for work every day and do what they're supposed to. Presumably your mentor can say that and more.