When placing any bet if better odds are avaliable when it reaches the market Betfair will ALWAYS match you at those odds. You will never be matched at worse.
So if you back at 3.2 you will not be matched at any odds lower but could be matched at odds up to 1000
If you wish to back at a lower odds or lay at a higher odds than the current price is trading at you will need to use our automation which will trigger your bet for you when the price reaches the odds you set.
For example if you wanted to back at 1.10 when the selection looks certain to win an automation file could be set up to trigger the bet when/if the odds trade at or below 1.11
One of the many example automation files we have avaliable to download from our forum has been created to do this and can be downloaded here