like brewski said do not mix any of them
if the system has DOT3 use DOT3 only

same with DOT 4 or 5
none of these intermix with each other very well and particularly the DOT5, it is synthetic so it will not mix with anything because of this.

And like brewski said if you want to change the fluid type you will need to clean out the complete system first before putting ANY of it in the system.

I haven't heard of DOT5.1 yet but it wouldn't surprise me if the engineers were coming up with a new fluid, they are good about that.