Thank you, Bill, for your comments and advise. It seems I have to live with it.
Yes, I am using Commander for rig control, which works just great.
With CAT selected as PTT method, keying my K3 is not steady. Sometimes K3 is keyed immediately when clicking Tune. Other times taking long time or even not. It might be because I am controlling K3 remotely with Remote Rig RRC 1258 setup. For some reasons, VOX does not work properly either. Currently I am PTTing manually in accordance with the TX of WSJTX. Really need some methods to key K3 steadily.