cannot use a mutable variable as an argument of the security function

For some reason, destructured assignments aren’t treated the same way when a user-defined function returns them vs when security() does. Encapsulating your security() call in a function will work:

//@version=4
study("")
f_sec() => security(syminfo.tickerid, "D", [high,low,close], lookahead = barmerge.lookahead_on)
[h, l, c] = f_sec()
h := h * 3
plot(h)

Note that you are using future data on historical bars when using lookahead and not offsetting the series by 1, as you are doing in there.