Not connected flight, i bought the ticker separately, is that ok? Spain - China - Thailand

Im spanish, i have already bought the ticket to chengdu, China, but hasnt bought the ticker from china to Thailand yet. If I bought the ticket from Chengdu to Thailand Now, and the flight will depart within 72hrs after I landed Chengdu, can I get the visa? The thing Im worry about now is it`s not a connected flight, so I may not get the confirmed seat no. for the one to Thailand.

Also, as Spanish we dont need visa for Thailand.

Do you think I can get the 72hrs visa free under this 2 conditions?

Thanks