I think similar topics have been discussed here but I haven't seen an answer to my question (yet).
We currently live in country A, my wife has passport B, I have passport C and our son has 3 passports (countries A, B, C). My wife will soon be travelling from A -> B -> A with my son but without me (potentially with a transit on a 3rd country). I may also need to travel A -> C -> A with my son but without her at some point.
I understand it's recommended to provide a notarised consent letter for every trip so that the other parent can travel without being stopped at the border. I also understand that bringing a marriage certificate and a birth certificate may help clarify if in doubt. In the end it's at discretion of the border officials.
Both countries B and C specifically offer notarisation of this consent letter in the consulates in country A. It also seems like the format of the letter is different in each case.
What country should notarise this consent letter? A, B, C or all three?
Country A: UK Country B: Russia Country C: Spain