Well, I'm not Theo, and not even very experienced, so feel free to take or leave this. I think the best thing would be to have a short chat with each of the three colleagues. Tell them that you are applying for the position as well, and that you wish them all the best in their applications too. Tell them that it's nothing personal, that this is simply a job you want to go for, and that you're sorry you didn't bring it up earlier as you felt uncomfortable doing so - you might even say that you weren't certain for a while if you really wanted the job or not. As three of your colleagues are applying, they must know that they're facing competition anyway. Just make it clear to them that this is not in any way personal and wish them luck.