How to get back certificates from company sir?

Sir i have done b.e(mechanical engineering) and after that completed pgdc(thermal power) at national power training institute neyveli.Myself and five other friends got placed in oeg india from campus which is a o&m company. At campus they recuited us for dcs(control room operator for boiler,turbine).But they made us sign a bond for three years(april14-april17) in a 100 rs non-judicial paper for 150000 rs.They said after 3 years in company they will post us at their gulf site operations.Since it was a campus interview and opportunity to go abroad after 3 years tempted me and my friends to sign the bond without any gurantor. In the bond they have said first 6 months plant familirasation with 7500 as gross pay.And after that next 6 months probation in dcs with 9500 gross pay.Upon completing one year will be promoted as desk engineer with applicable salary and benefits.(21000 aprox)They posted me and my friends to different sites. At the site they put us at coal handling plant immediately in shifts and said after two months they will shift me to boiler or turbine field and will give training for dcs.I worked continuously for six months in coal handling and when i requested them to put me in control room for training they said they would not be able to do so.Hence i contacted my head office and requested them to shift me to control room for training.But unfortunately they too did not respond positively.I tendered my resignation in registered post and through email in sep 2014.They threatened me pay the amount.Hence i employed a laywer and we sent a legal notice and after that they are not responding.Two of my friends have paid full amount and left the company while other two are still working in field with a pay of 12000.Sir during this time i have cleared prelims in tamilnadu public service comission and going to take mains.I would like to go to work in private sector as a backup option but i dont have the certificates of(10,12,engineering degree) as they have taken it.Please suggest me a way sir to get back my certificates without paying 150000.I am willing to pay the salary they have given in six months which is 6800 after deducted esi and pf of 40000.