To get them easily u will need:

glalie
gengar
raichu
medicham
full discharge
full styler

To catch Raikou all u need to do is that after he uses thunder 4 times use poke assist with glalie. To catch suicune, when u know the real one use poke assist gengar And to catch Entei, after he has no flames on his body use discharce after that use medicham to catch it faster. Only use the raichu when battling Entei. I hope this helps. With this i beat the game in on August 17 2007 at 12:01 am. I beat the game in 10 hours and 51 minutes