In Engineering it's a threesome (Score 1)

It depends on the field as tuxidriver said. In engineering I see all three OS. In computer science it's mainly Linux and OSX. In Electrical Engineering (Digital Design, ASIC, FPGA, etc) it goes from Windows to Linux mainly, but a lot uses OSX and connect to the Linux or Sun server to run their tools (as I do, when I'm not on windows). In the Physics Department, they are wild on OSX (especially Nuclear Physics), some computer run Windows when capturing data since the captor driver are for windows only. In Mechanical they use Windows and sometime Linux. For the rest, I don't know but it seems to me to be mainly Windows. There is no good answer, but Windows and Linux are the best bet for now, if they want to use some specialized tool. In a lot of case (Electrical Engineering) the tool are develop on linux and ported to windows. So those tool are way more powerful on linux. But the reverse is true for Mechanical. Ask what they want to do and from there you can choose a better answer.