1. MTR + Airport Express very marginal, consider longish walk from HKBU to Kowloon Tong / Lok Fu + zillion stops from there to Tsing Yi
Tsing Yi : 67min+ (12 min to walk to MTR, 3 min to wait for train, 25 min to Tsing Yi, 12 min wait train for airport express, 13 min airport express)

2. Taxi to Kowloon Station is way too risky, at 3:20 the traffic starts building up from the Harbor Crossing / TST and congestion is waiting for you. I have had case that West Kowloon is totally congested and I have to spend 1 hr from Olympic station to TST on car. In your case you also back tracking. It takes 51 min in total + possible crazy traffic (18 min Taxi, 12 min max wait, 21 min train), compare to 45 min for Tsing Yi option.

3. Taxi all the way / Taxi to Tsing Yi + AEX is the only feasible options as
3.1. It goes via Lung Cheung Road / Ching Cheung Road (the pseudo highway connecting East and West Kowloon)
3.2. Then it connects to Route 3 and 8, nothing can go wrong