Each group of three definitions describes three words that are spelled the same, except for one letter (each group describes a different set of words). Example: king, ring, wing.
1a) competition
1b) satisfied
1c) the circumstances surrounding an act or event
2a) to rain hard
2b) sullen
2c) a number
3a) a tartan skirt
3b) a heated enclosure used for drying
3c) to deprive of life
4a) to contaminate
4b) to force into something
4c) flea