I'm pretty sure with most of these fluids you can mix them,
In this case, the term "mix" does not apply.
You can pour DOT-3, DOT-4, DOT-5, and DOT-5.1 into the same container and shake them until the cows come home. While the DOT-3, -4, and -5.1 fluids will mix and stay blended, the DOT-5 will separate out. Think of this as "oil and vinegar". They are both liquid, they can sit in the same jar, but they won't stay mixed. The fluid types are too different.