As usual; sales people don't know what they're talking about (or explain things poorly, or were poorly understood).
"Normal" ADSL (which in this country is ADSL1 - Or "ADSLmax" as BT call it) can go up to about 8Mbps downstream connection speed (and around 800-900kbps upstream).
A typical BT Wholesale product will be "upto" 8Mbps down and 448kbps up.
Sky's own version (if you're lucky enough to live in an area where they've bothered to install their own equipment in the BT exchange) will be similar but may have a different upload connection speed.
The "only go up to 3Mbps" is probably referring to your line length data and the estimated speed for the length of line (or at least how long BT think it is - which is often wrong).
As you're 2.5kms from the exchange, your line length could be anywhere from 2.5kms to maybe 7 or 8kms, although a typical figure would be 1.4x the direct distance (so around 3.5kms).
Anywhere between 2Mbps to 6Mbps (depending on the actual length of your line) would be possible.
In practice; you'll automatically get the fastest speed your line can support (it's unlikely to be capped at 3Mbps just because that's what the estimate came out at).
Ade
ADSL2+ with BE
DL Sync Blimey....4707kbps now!
UL Sync 1088kbps
DG834GT with DGTeam firmware