Use 3 optarg in two functions bash script

You need to change two things:

1. Since the -h option does not take any arguments, your first argument to getopts should be hi:b: (or b:hi: if you want it in alphabetical order). That is, options that take an argument should be listed with a trailing :, and those that don't take an argument should be listed without a trailing :.

2. You probably want exit after calling your help function, otherwise, the code would continue with calling sniff later.

I would personally also document the -h option. If there was some command line parsing error, I would call your help function but exit with exit 1 afterward in a *) catch-all case at the end of the case ... esac statement.

#!/bin/sh

cmd=$(basename "$0")

if [ "$(id -u)" -ne 0 ]; then
    printf '%s requires root privileges\n' "$cmd" >&2
    exit 1
fi

help () {
    cat <<END_HELP
Usage:
        $cmd -i interface -b mac_address
        $cmd -h

        -i      Wireless interface name (monitor mode)
        -b      BBSID wireless mac

        -h      Display this help text and terminate
END_HELP
}

sniff () {
    airodump-ng "$1" -w PSK -c 36 --bssid "$2"
    err=$?
    if [ "$err" -ne 0 ]; then
        echo '[!] Error. Check that SSID name is Radius and channel is 7' >&2
    fi
    return "$err"
}

while getopts hi:b: opt; do
    case "$opt" in
        h) help; exit ;;
        i) i=$OPTARG  ;;
        b) b=$OPTARG  ;;
        *) help; exit 1 ;;
    esac
done

sniff "$i" "$b"

As there is nothing in the script requiring bash, I turned into a /bin/sh script. The test of EUID is more safely done with id -u as it's easy to otherwise set EUID=0 in the script's environment to bypass the test.

Rather than exiting in sniff I let it return the exit status of the airodump-ng utility. The main part of the script would then exit with that as its exit status as sniff is the last command in the script.