Holiday with friends dilemma

Hi, so me and my 2 besties really want to go on a holiday but when we find a time that fits all of us one friend always lets us down by something coming up or no money to book. Me and the other friend went away on a weekend break earlier this year but that was only because my boyfriend could no longer come with me and I asked both friends and she was the only one who could do it.

So we all agreed we could do August and this was a compromise from my end as I have a lot or money coming out in August for other stuff and now this friend can not get it off work but I’m not going to get a holiday all summer and my other friend still wants to go.

Would it be unfair to go without the other friend?

How do we tell her we still want to go without looking bitchy?