Jelly, 05AB1E, 6 bytes, score = \$\frac 6 8 = 0.75\$

.Vq ɠV 

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Previous answer that does 3 languages:

Jelly, Python 3, 05AB1E, 29 characters, score = \$\frac {29} {27} \approx 1.07\$

exec(input( ))#}I.Vq 1 ,1#ṛɠV 

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

Jelly, 05AB1E, GolfScript 8 bytes, score = \$\frac 8 {27} \approx 0.296\$

.Vq;~ ɠV 

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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.

Jelly, 05AB1E, 7 bytes, score = \$\frac 7 8 = 0.875\$

I.Vq ɠV 

Try it online Jelly!

Try it online 05AB1E!

Previous answer that does 3 languages:

Jelly, Python 3, 05AB1E, 29 characters, score = \$\frac {29} {27} \approx 1.07\$

exec(input( ))#}I.Vq 1 ,1#ṛɠV 

Try it online Jelly!

Try it online Python 3!

Try it online 05AB1E!

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

Jelly, 05AB1E, 6 bytes, score = \$\frac 6 8 = 0.75\$

.Vq ɠV 

Try it online!

Try it online 05AB1E!

Previous answer that does 3 languages:

Jelly, Python 3, 05AB1E, 29 characters, score = \$\frac {29} {27} \approx 1.07\$

exec(input( ))#}I.Vq 1 ,1#ṛɠV 

Try it online Jelly!

Try it online Python 3!

Try it online 05AB1E!

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

Jelly, Python 3, 05AB1E, 29 characters, score = \$\frac {29} {27} \approx 1.07\$

exec(input( ))#}I.Vq 1 ,1#ṛɠV 

Try it online!

Try it online Python 3!

Try it online 05AB1E!

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

Jelly, 05AB1E, 7 bytes, score = \$\frac 7 8 = 0.875\$

I.Vq ɠV 

Try it online Jelly!

Try it online 05AB1E!

Previous answer that does 3 languages:

Jelly, Python 3, 05AB1E, 29 characters, score = \$\frac {29} {27} \approx 1.07\$

exec(input( ))#}I.Vq 1 ,1#ṛɠV 

Try it online Jelly!

Try it online Python 3!

Try it online 05AB1E!

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