on embedded Linux you typically need to have to full source code in order to compile the libraries of the SDK for your target system. All three vendors you mentioned have offerings for embedded Linux and ARM CPU. Therefore you deicsion of a proper SDK should depend on other criteria like ease of use and in depth functionality or the size and speed that your application desires.
You should give all the three a test, When looking at the functionality you can evaluate that on their standard (windows based) demo and evaluation offerings. At Unified Automation you can even get a binary precompiled Raspberry PI evaluation edition (which is an embedded Linux on an ARM), so it might be pretty close to what you are looking for.