I plan on buying a home about a year from now.

I'm 25, have never owned a home before, but I have a credit score that hovers around 780, so I've got that going for me. However, I do not plan on co-signing and my credit history will be 5 years long (and a few months) when I plan to buy.

Some friends of mine recently attempted to get a mortgage to buy a house, but were denied. I ask this question because my friend says they were told they'd have been approved if "I had a third line of credit at least a year old."

I have two open credit cards (which I pay off) and an auto loan, which make up my credit history. Is there anything significant about the number 3 such that I should be looking into a third credit card? Especially since I'm purchasing alone? Is it more about the total amount of credit available to me?