I've never seen anybody able to get rid of large amounts of clover, crabgrass and dandelions. I'm not saying that it's not possible, but I've known a lot of people who've put a ton of work into eliminating this stuff (including me) to see little improvement. I live in an older neighborhood in Rochester and whenever I see someone with a particularly good patch of grass it's usually because they tore up their mess and started from scratch.