Well there are only a few reasons that the speed would drop.
1) There is a fault with the mast.
2) There is a fault/capacity issue with the backhaul.
3) The bandwidth is being used up by other users, simple contention.
4) Three have changed something, i.e. channel width and or Tx power that now means the speed is lower for your location.
Either way you have no chance in hell getting Three to admit what the issue is or that there is even an issue.
There used to be a option to report a fault through the coverage checker, but I can't see this any more.
To be honest with 3G it is pot luck. It is all too easy for a mast to become over loaded (contended) as Three have no control over how many customers might sign up for their service in the coverage area of any one mast. Speeds are often rubbish in towns and cities as the masts are overloaded. Where I live I'm lucky to get a 3G signal even though Three claim indoor coverage. And in locations with full signal it only manages 0-5~0.7Mbps. My usual blame for this is the student population... Out of town on a rural mast I have seen speedtests for 26~30Mbps on DC-HSPA.