Final answer:
The term for when a neighbor's fence extends onto someone's property is an encroachment. Encroachment involves property law and may require legal action to resolve.
Step-by-step explanation:
When a neighbor puts up a fence that extends three feet over onto your property, this is referred to as an encroachment. Encroachment is a term in property law that describes a situation where one's property extends onto that of another without permission. Resolving such an issue typically involves legal action where you may need to ask the neighbor to remove the fence, or potentially pursue a court remedy to enforce your property boundaries.