You're doing very well to have two and potentially three offers in the bag, Smores.
I've just called the employer to accept without any further mention of this interview. They are sending out a written offer to which I must reply with my acceptance in writing. I'm going to stall as much as I can and hopefully get the other job.
Smores, could you start the July job, accept the September one and still go to the latest interview? Would it be possible to take your pick like that?