Can I apply now if I will enter over 3 months away?

I'm currently in Manchester so will be able to hand in the application at the office but I don't know my exact date when I will be entering China and is likely to be over 3 months away as I am leaving this month and travelling for the next 4 before flying out of Hong Kong. I will be entering mainland from likely Vietnam is it better to apply for a Visa here and try and get there in the next 3 months or will I be able to apply for it whilst in Thailand, Cambodia, Laos or Vietnam?