Function testbench::concurrent_test_3

pub fn concurrent_test_3(
    f1: impl FnOnce() + Send + 'static, 
    f2: impl FnOnce() + Send + 'static, 
    f3: impl FnOnce() + Send + 'static
)

Test that running three operations concurrently works

This is a variant of concurrent_test_2 that works with three functors instead of two. It is hoped that future evolutions of Rust will render this (light) code duplication obsolete, in favor of some variadic design.