These are quines in the programming language MUMPS. Just to show that you can.
Actually, you can very well. MUMPS, while not designed especially for quines, is from the era when self-referencing (and self-modifying) code was a given, so it has numerous primitives that make it remarkably easier to write quines: eval (called "XECUTE"), expression eval (called indirection), introspective facilities for source code (called "$TEXT") and so on.
In its own way, MUMPS is quite alike today's scripting languages, but like most other languages of its era, has weird quirks such as space-sensitive syntax and commands you can abbreviate. And some primitives like string substitution are sorely missing, as are efficient data structures and proper function calls.
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Quine 0 is a quine because many MUMPS implementations output nothing when they execute an empty source file.
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Quine 1 cheats by using the introspective $t function to read its own code.
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Quine 2 leverages the fact that you can dynamically execute code from strings with the X command. Then the program can read itself from the same string it is being executed from.
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Quine 3 is a "traditional" quine in that it doesn't use any language-specific quirks. It just reconstructs itself from a string containing its textual representation.