Is it legal to trade 1 wood for 1 wood + 1 hay?
I would argue the rules say this is not legal.  Quoting from rules found on Catan Website


  However, you cannot give away cards, or trade matching resources (“trade” 3 ore for 1 ore, for example).


Your example in effect breaks both those rules.  You are trading wood for wood which is illegal.  The exact amounts don't matter, the example is 3 and 1 ore but if there is a matching resource in both sets then it is not a trade.

If you take out wood cards from your trade then you are in effect giving away a Wheat card for free which is also against the rules.