That means, that you need to specify one more column in your using
statement: The first one is the x
-coordinate, the second one the y
-coordinate. The one for the variable line color is missing.
Use e.g.
plot '/tmp/data.csv' using 3:1:0 with points pt 1 lt 3 lc var
to use the row number (zeroth column) as linetype
index. You can also use e.g. linecolor palette
so select the color from the currently defined color palette.