curving river radical (no.47)
巛 represents a curving river or flowing water, used primarily as a radical in other characters rather than as a standalone kanji.
巛 is a pictograph of a winding river or flowing water, with the three curved strokes representing meandering streams.
The three curved lines look like a river winding through a landscape: 巛 is a curving river.
For セン, picture a thousand (千, せん) tiny streams merging into one curving river: 千 -> セン, and the river bends through the scene.