First last and security deposit

I'm finding a lot of people can't come up with three months deposit; plus I'm wondering if they have a good background check; verify job;no evictions; or criminal background; is it OK only to charge first and last month and maybe 2 to 2.5 income to rent ratio 

 after studying Brandon's book I can't seem to find those types of tenants that are described in his book 

1ST Three times the income to monthly rent

2ND three months down deposit $

that's the question that I'm fine it hard to find a tenants