They're very, very careful and want to see it ALL. I've heard that typically your middle FICO score is used.
SOMETIMES you can do a NO DOC/NO CREDIT REPORT LOAN if you have enough EQUITY on a RE-FI or a VERY LARGE DOWN PAYMENT on a new purchase (like 40% or more down)... ...AT HIGHER RATES (usually)
they will always pull all 3 credit reports as it will more likely be part of a larger portfolio to be sold. the criteria the lender will ask you for/about will depend upon the investor. lowest middle score if there are two borrowers will be used to determine the rate; except in the case of high LTV (>95%) in which case it will usually be the middle score of the primary income borrower (the person with >50% of income). based on that a 620 will get you a conventional mortgage...660 will get you a higher ltv loan...700 score will allow for no doc/no asset programs and 103% ltv loans. in addition to score, a high amount of liquid assets (amount readily available in an emergency, such as savings and checking) will help balance a lower score. you are better off getting all three bureaus since it will give a better chance at getting a higher middle score. if your worried about bad credit, many lenders allow collections, etc, as long as they aren't more than $250 each or $1000 total. my recommendation would be to fix all your errors before obtaining a mortgage (save you refi costs later). if there arent errors and simply poor credit, either wait until it drops off or negotiate with creditors, but I would recommend waiting at least 2 years since your derog in order to qualify for decent programs.