Dot 3 is dot 3, dot 3/4, dot 5.1, are all a glycol based fluid. All have different heat ratings and boiling points.
What you do NOT want to do is mix synthetic (3, 3/4, 5.1) with silicone based fluid (DOT 5).
DOT 5.1 and DOT 5 are NOT compatible, even though the numbers are similiar.
And as an FYI, DOT 5 cannot be used with ABS equipped trucks, the rapid pulsing of the pistons can actually make the fluid "froth" and may create air in the system.
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