From a language point of view, this sentence can be parse in two ways:
1) You can apply for a visa up until a moment in time that is three months before your travel date.
2) You can apply during a period ranging from 0 to 3 months before your travel date.
Now, purely practical, the only interpretation that makes much sense in this context is the second one. Otherwise, nobody could apply for a visa unless they plan their trip more than three months in advance - and strict immigration rules may be one thing, but things like business trips (that stand to make the UK money indirectly) often come up on a shorter notice.