Can 3 Monitors Be Connected To a Mac Pro (late 2013] ?

Can 3 monitors be connected to a Mac Pro (late 2013]? I can get two monitors working with no problems, but when I try to add a third monitor it doesn’t get a signal – turn on. I’ve tried everything; it’s not the cable, and it’s not the busing or plugging array because I’ve tried every possible combination. The third monitor will work if I pull out one of the other two. (All Digital input.)
I’ve come to the conclusion that the mac is just not capable of driving three monitors.
Is there something I could do that would make it possible to drive the three monitors? Can I update the GPU in a Mac pro, or something like that?
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Joseph Renzetti