Save as /etc/polkit-1/rules.d/40-custom.rules and RESET i3!

40-custom.rules

/* -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*- */

// Custom user-defined rules for polkit
//
// See the polkit(8) man page for more information
// about configuring polkit.

polkit.addRule(function(action, subject) {
    if (action.id == "org.freedesktop.login1.suspend" ||
        action.id == "org.freedesktop.login1.suspend-multiple-sessions" ||
        action.id == "org.freedesktop.login1.hibernate" ||
        action.id == "org.freedesktop.login1.hibernate-multiple-sessions" ||
        action.id == "org.freedesktop.login1.reboot" ||
        action.id == "org.freedesktop.login1.reboot-multiple-sessions" ||
        action.id == "org.freedesktop.login1.power-off" ||
        action.id == "org.freedesktop.login1.power-off-multiple-sessions" ||
        action.id == "org.freedesktop.login1.reboot" ||
        action.id == "org.freedesktop.login1.reboot" &&
        subject.isInGroup("power")) {
        return polkit.Result.YES;
    }
});