For as iconic as these characters are, a group cosplay of the three main characters from the Pokemon TV show is criminally easy.
If you're a redhead (and a Pokemon fan), a Misty cosplay is a no-brainer, and you likely have everything you need already. A yellow crop-top tank, cuffed jean shorts, and a pair of red suspenders are all you'll need to take on the role of Cerulean City's favorite gym leader. Oh, just don't forget a hair tie for that cute side ponytail!
This is another very simple costume that you can largely put together with things you already have in your closet (or from the local thrift store). All you really need is a pair of brown khakis or work pants, an orange shirt, and a green fishing vest. Extra credit if you wear a backpack as well, but it's not necessary.
Ash isn't that hard to put together, per se, but the person in your group cosplaying as the character will likely have to buy a fair amount of stuff that they don't already own. The base of a black shirt and light blue jeans are easy enough to find, and most of you will have a backpack laying around that you can use as well. The top, hat, and gloves, however, will likely need to be sourced elsewhere.
Luckily, eBay has a plethora of bundles that include the jacket, hat, and gloves all in one package for a decent price! In addition, Amazon has a pretty nice version of Ash's Pokemon League hat as a standalone item available here.