A friend wanting to travel to Canada was asked for a proof of $3,000. Usa funds as a declaration fee. I this legal

Nope, this is a SCAM!!

I don't know Canadian immigration, so I recommend calling Citizenship and Immigration Canada or an experienced Canadian immigration attorney.

It sounds like a similar requirement for US visitors to have funds to pay for their stay as well as evidence that they are not going to overstay and work unlawfully in Canada. Doesn't seem like a scam in that they are not asking for money, only evidence of the funds.