Do the level crossing barriers near Hythe Tesco, Colchester, only ever stay down for a maximum of two trains

They stay down for as long as the signaller deems it necessary in order to minimise delays to trains. Strangely enough road traffic is not really taken into account - and indeed the signaller often can not see what is waiting at the crossing anyway.

So sometimes your barriers could be down for just one train; other times three or even more trains. It all depends on what is going on at the time.