Do not mix DOT 5 with any other brake fluid. DOT 5 is silicone based; DOT 3, 4, 5.1 are glycol based. Mixing 5 with any glycol based fluids will result in a gelled mess with no braking. DOT 4 has a higher boiling point than DOT 3, but 4 absorbs more water and needs be changed more frequently. Same goes for 5.1 over 4. DOT 3 is probably fine, but all brake fluid absorbs moisture and needs to be flushed. If you really want to use DOT 5, you will have to totally and completely flush out any traces of DOT 3.
