So its good for changing fluid in a bike that you do not know the grade of fluid is in there already as there will always be remnants of the old in there --- is that right ?
Yep........ Unless it's DOT5 this you wont or shouldn't come across on bikes, unless some nit has used it by mistake. 5.1 is a much improved version of 5 and is compatible with DOT3 & 4 without it's handling issues. DOT5 which is silicon based and handled more heat less hygroscopic, than 3- 4 but has handling issues I wont bore you with the details look it up if you're minded. Just don't use it.
Also what use is DOT 3
It's not used much these days for drum brake systems that operate at lower temperatures, it's just has a lower boiling point. All bike and cars for years have been DOT4 many are 5.1 nowadays.
A few were made with 5 but most can be converted to 5.1 but require complete system strip and clean beforehand they can't be mixed.