What's the inventory locking behavior of the system and what is the activity level?

I'm wondering whether it could be the case that there is enough "activity" in the system to cause it to have only two spots left in inventory - for example:
1. The Q inventory has three seats available.
2. Someone has one spot in the Q inventory locked up in a transaction. This must be pretty common.
3. Your query for three seats only finds two seats available in Q and has to bump one passenger up to V.
4. You say, hmm and do the thing as three separate transactions. By the time you get around to the third, the person in #2 has decided to cancel their purchase and the inventory is available.

Does everyone in a party have to travel at the same fare basis on all legs?