If you're number three, then the seller is thinking that they may be getting some better information or a new approach from one or more that are arriving after you. If you're last, you're better able to address any remaining concerns, as they may be more prone to divulge them knowing there's nobody left to ask.
It's not always possible, but it's worth a try to say "Could I please be your last appointment? I'm a bit different from most REALTORS®, and I want to be able to address any questions or concerns you have after talking to others."
