The use-in-context of three lexical units - at, on and in - is analized in thousands of examples from the BROWN corpus of written American English. Syntactic scrutiny confirms the lexicon-grammar continuum, as far as particles cannot be categorized according to necessary and sufficient conditions. Semantic analysis is performed according to a polysemy model based on prototypes and radial categories. It is shown that the same semantic structure is common to all the uses of these particles, as a result of partial sanction and semantic extension of a single category. A prototype is posed for each category, based on topological, force-dynamic, and functional experiential dimensions, according to human perception of objects and their relationships. Particularly, the semantic structure of AT is developed on the basis of an ENCOUNTER schema, that of ON follows from a SUPPORT schema, and finally, that of IN derives from both an INCLUSION and a MEDIUM schema.