Arcanine is a pretty decent fire type.
HGSS doesn't really have many Good grass types, so any you can get your hands on would probably work.
Poison.... You've got Crobat, Beedrill (which isn't to bad in Gen IV and up.), Gengar, Tentacruel (if you don't already have a water type), Venomoth.... that's all I can think of.
Or you could go for Victreebell and get grass and poison in one shot.