Function merge_opt_i3 

pub fn merge_opt_i3<I1, I2, I3, F, O>(
    I1: Option<I1>,
    I2: Option<I2>,
    I3: Option<I3>,
    func: F,
) -> Option<O>
where
    F: Fn(Option<I1>, Option<I2>, Option<I3>) -> Option<O>,