Those pistons you have fitted are not stock they are forged pistons (cp I would say, markings on underside). As said there are cracks in the head and no3 has oil in it as well as the combustion chamber in head so definitely an issue with that cylinder as that was oily prior to dis-assembly.
Don't waste your money getting the head welded as it will not last I assure you (speak from experience, ive around 20 heads here which are cracked and cracks can travel deep when hot), up to you but really needs replacing.
You wont know all until you pull the pistons but you may just get away with a rehone/deglaze then fit new rings assuming cylinder ovality check with dti comes out ok and within spec.
Now on another note, when does your lad want a job
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