Confused me too, Mark, it seems a-about the way they put the odds up. Must be a pommie thing, like Ritchie saying it's 222/2 at Lord's.
Reiterating becareful, there is no 3.50 on offer in your example, that is just the current low price a bettor is prepared to accept and 3.00 is the highest price a layer is prepared to lay.
There is a standoff till one meets the other at one end or somewhere in-between.
Once you get on and start pressing the buttons you soon get their drift.