I now see Route 3 -> Route 6 is using motorway; this makes sense because we have precedent for this in Route 5 -> Route 7, which contains an exit for Route 5 itself and also provide the (branching) starting point for Route 7.

I see Route 6 -> Lin Cheung Road is also using motorway.

Upon rethinking, I have no real proposals/ideas for the Lin Cheung Road -> Route 3 part. If anything, this can be revisited in the indeterminate future when highway reclassification happens.

Then, nothing to do here. Closing.