You could explain what the $3,000 is for and why you need it.  It seems like someone with past development experience wouldn't need this sum of money to create a site that does what you are seeking to do.  That makes me think that you or your developer lack the faith in your brainchild and want to be paid up front for your development time to build what you want.  

I apologize if you feel this is trolling.  I am just trying to share my thoughts with you.  I think developing the site first and then seeking investors once you have advertiser or user data would be a better way of going about it.  Many new users come here looking to scam.  Unfortunately the burden of proving your are legitimate falls on you and without a product it will be difficult to convince people that you are not just seeking a free $3,000.