Dubrovnik to Mostar to Sarajevo and back

For clarity, did you mean arrive into Dubrovnik on August 23rd at 5:00 pm and on that same day you would like to get to Mostar and then to Sarajevo to sleep that night of the 23rd? At any rate, you have essentially a long weekend and 3 countries you'd like to visit. I would suggest you pare that down to about 2 places to see, ideally geographically compatible places with good transportation connections. Otherwise, your entire trip will be spent on a bus! Of the places you mention, do you have one or two places you absolutely wanted to see?

Having traveled to Dubrovnik, Montenegro, Mostar and Split (although not Sarajevo), I can tell that visiting all 5 of those locations in such a short timeframe will be very challenging. I don't think it can be accomplished using public transportation with adequate time left to sightsee. It might be possible with a rental car, but even that would be tight. I'd suggest focusing on fewer locations, but if you decide to visit all 5, put Montenegro at the beginning or end and not in the middle. A lot of time will be spent backtracking if it's in the middle.

The line waiting to cross the border from Croatia to Bosnia is pretty long. I think you're trying to do too much based on that alone. Try spending the night in Mostar and save Serbia for later.