Remember to include information on where you live as it makes a big difference if you live in Boston, USA or Harare, Zimbabwe - the faunas of different parts of the world are very different. Telling these three groups apart is relatively easy. Most katydids are green and have long wings. Crickets are usually brown or black, and like katydids have long, thin antennae. Female's ovipositor is thin, needle-like.
Most crickets are stout and more compact than katydids.
Grasshoppers have short, relatively thick antennae, which are rarely longer than half of the body. Female's ovipositor is short, often barely visible. Most common grasshoppers are brown or olive-green. Please remember that these descriptions are extremely simplified and that there are many members of the three groups that can be easily confused with others. Most katydids can be categorized as opportunistic omnivores.