That would return 20b, 1, 0, you need to start from the second thing in the table to the last because the identifier is the first thing it would put in the table, so it has to be
local coords = "20b_1_0_-1"
local values = string.split(coords, "_")
--or
local values = coords:split("_")
print(values[2], values[3], values[4]) -- 1, 0, -1
--Or if you just want the coords only, ignoring the identifier thing,
local values = string.split(coords, "_")
table.remove(values,1) --Remove first thing in table
print(values) -- {1, 0, -1}