Well, you can plant corn and beans with a corn planter but you can't plant grain. If you only have money for one implement to do three things then go with a decent drill that list corn and beans on the rate chart.
For putting seed in the ground I have a few options. I have a couple drills, a 4 row corn planter (used 4 rows are usually cheaper then 2 rows), a Brillion seeder (great for small seeds) and a couple Vicons that will put seed down as well as fertilizer (just need to run a packer over the field after the seed is down). I don't use any one particular unit more then the other as they all do their jobs and do them well.
