Register GET REST API route with multiple parameters

You don’t need to include query parameters in the endpoint. Just the path:

register_rest_route( 'myplugin/v1', '/posts', array(
    'methods'             => 'GET',
    'callback'            => 'my_rest_function',
    'permission_callback' => '__return_true',
    'args'                => array(
        'number' => array(
            'validate_callback' => function( $param, $request, $key ) {
                return is_numeric( $param );
            }
        ),
        'offset' => array(
            'validate_callback' => function( $param, $request, $key ) {
                return is_numeric( $param );
            }
        ),
        'total' => array(
            'validate_callback' => function( $param, $request, $key ) {
                return is_numeric( $param );
            }
        ),
    ),
) );