Can I have a conditional shebang?

Not directly, no. Best to write a wrapper Bourne shell script and shebang that:

#!/path/to/my/wrapper

and the wrapper starts with:

#!/bin/sh
for shell in first second third; do
    if /usr/bin/env "${shell}" "$@"; do exit $?; done
done
# We didn't find any of them.
exit 1

This lets env(1) use the ${PATH} search list to locate the programs, in the order given in the for-loop.

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