Yes, If you cant afford the loan, make it a point to your lender and ask for a letter stating they will not fund the deal. You should be able to walk away with your deposit since it is a contingency on most contracts. The sooner you get this to the seller, the less of an issue it will be.

Marcelo,
If you are 3 weeks from closing, then you probably have not received a mortgage commitment from your lender. Talk to your loan officer about it. If you are rejected by your lender, then you can get out of the contract without losing your EMD. Is this a condo? There is also a contingency to review condo docs that could allow you to get out of the contract without loss of deposit.
If you are already past that, talk to your agent and/or your lawyer about what other escape clauses are still available to you in the contract. If you have no other escape clauses you can invoke, then you only risk the deposit monies you have given.
I would advise you to act quickly. If you can't complete the sale, it's only fair to the seller to let them know as soon as possible, as their house is now off the market for you. The more time that it is off market "for nothing", the less likely they will be to work with you.

Marcelo,

Since you are dealing with an executed contract your best action is to seek the advice of an attorney. They will be able to review the document and provide a course of action.

Good luck,

That depends on your Purchase and Sale Offer. Is there a financing contingency? Were you approved for the mortgage? If so, then you wont' be able to walk away because of the financing contingency. If your mortgage is declined, and you notify the seller within the contractual time frame (usually 30 days), then you should be refunded all of your escrow money. Did the house appraise high enough for the loan to be approved? If not, that is another way out.

What about inspections? Was this an "as Is" sale, and you can decline to purchase based on unacceptably high costs for repairs?

You really need to talk with your Realtor but be sure that the Realtor represents YOU. If you do not have a Realtor, consult your attorney for advise. Your attorney can read the contract and advise you appropriately. At any rate, you need to act quickly because time limits are crucial.

If these contingencies and any others have already passed by, then you may lose all of the escrow money. Good luck.