Three do operate a priority system. Everybody does.
3 have multiple priority systems based on type of data and the type of customer - mobile broadband customers get data priority over non mobile broadband. P2P and tethering is also throttled. PAYG is AFAIK not a distinction the system cares about.
The start date for the contract will be the day the SIM becomes active, which will vary depending on whether you bought the contract in a store or online. In-store purchases will activate straight away, online orders vary depending on where you bought it from but typically will be the date of
expected delivery.
The actual billing date however will be completely different. The billing date is the date your bill is generated as well as the date your call and SMS allowance is refreshed. This is "up to nine days" after your contract starts and your SIM is activated:
http://support.three.co.uk/srvs/cgi-...=obj%283620%29
Most important things - allowances, add-ons, and of course payment - will revolve around your
billing date rather than your
activation/contract start date. However cancelling the contract is based on your
activation date not your billing date though in this case it's not really relevant, since you can cancel (well, give 30 days notice) from day one.