How we can calculate the %total acidty(as in oxalic acid, sulfamic acid and formic acid mix) in terms of sodium oxide.

I hate to see questions go completely unanswered -- we don't just ignore them. There seems something fundamentally wrong with this question. Often I have seen analyses of related compounds in terms of a typical standard: "carbonates as CaCO3" or 'Phosphorous oxides as P2O5". So to analyse three very different acids in terms of a base makes no sense. Maybe you need to titrate the mixture, calculate the moles of NaOH consumed and convert that to moles of Na2O. (Why, I wouldn't know)