Electro - Force Redux Framework to 3.x

functions.php

function ec_child_tgmpa_plugins_force_redux_3( $plugins ) {
    foreach ( $plugins as $key => $plugin ) {
        if( isset( $plugin['slug'] ) && $plugin['slug'] == 'redux-framework' ) {
            $plugins[$key]['source'] = 'https://transvelo.github.io/included-plugins/redux-framework.3.6.18.zip';
        }
    }
    return $plugins;
}

add_filter( 'ec_tgmpa_plugins', 'ec_child_tgmpa_plugins_force_redux_3', 20 );