3% vs 40% mortgage interest

[William Walker]

I often see people post asking if they should pay down a mortgage or their student loans/car/credit card. And often times a reply says something to the tune of pay down the credit card because the interest is +10% and the mortgage interest rate is 3% or whatever.

When I sign for a mortgage I get a % such as 3%, but I also get a % over the life of the loan, something like 40%. So if I pay the minimum payment every month for 30 years, the total % I will pay is 40%. I buy a house for 100k, I pay 140k.

Explain to me in simple terms why I should look at a mortgage as 3% and not 40%. Much appreciated.