Function access_all_3 

pub fn access_all_3<A, B, C>(
    a: &GpuRef<A>,
    b: &GpuRef<B>,
    c: &GpuRef<C>,
) -> Result<(Arc<CudaSlice<A>>, Arc<CudaSlice<B>>, Arc<CudaSlice<C>>), GpuError>

Validate three input GpuRefs and return owning Arcs of their underlying slices.