OP - there is no publicly available landside connection or service that allows travellers to freely go between T1 to T3 or T3 to T1 traveling on any airline to any airline in their respective terminals. You have to go ground side using the terminal link train service.
The ground side terminal link service is 24/7 although during slow hours only one of the two trains are running.
Last edited by yyznomad; Aug 12, 2017 at 8:29 am