Polyglot self-interpreter time
Jelly, 05AB1E, GolfScript 8 bytes, score = \$\frac 8 {27} \approx 0.296\$

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Try it online Jelly!

Try it online 05AB1E!

Try it online GolfScript!

In all cases, takes a program in the relevant language from STDIN and executes it, with any output going to STDOUT.