I am a fresher completed my comp engg.I gave interview in a company in june and while interviewing t

I am a fresher completed my comp engg.I gave interview in a company in june and while interviewing they informed me that the joining would b in july first week.They sent me the offer letter via mail and asked me to sign it and mail it back to them if i accept the offer which i did.But then i was informed that the joining would be in fourth week of july,then after that when about a week before my joining date they postponed it to second week of august.Still they have not given me an exact date and they are saying i cant go back on my offer.Now i dont feel like joining this company because i declined two offers since i thought joining was in july,and i dont feel that a company who cant decide a joining date for its candidate is a good place to work at. The offer letter stated that if i sign the letter it means that i accept to work for a period of three years failing to do so i will have to pay them some amount which is more than a lakh. So what i want to know is that whether i can walk out of this offer since i have'nt joined yet.They have my original documents (marksheets).What should i do if they dont give me my certificates back.