Prem will marry to agree BECAUSE he has already lost heer and chahat now he would not want to loose his mother either ... its only because of emotional black mail SO RELAX!!!
he doesnt that to make g3 happy he will loss a little chance of get heer specially chahat back ? yah during ashlesha time he agreed bcoz he knew heer is dead ,this time he knows chahat & heer r alive ,i lost heer but prem agree to marry bcoz of g3 will ruin him ,if its bcoz of heer reason ,then i wont have problem