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