The poor little thing is only 3 mo! My reaction would be to hold and cuddle all day long.
But knowing it's not possible, maybe you could supply a sleepsack for them to try? Some babies do really well with them for sleeping purposes.
Certainly, they cannot expect your baby to adapt or change that quickly can they? I could understand their feelings if he was older but at 3 mo, he should be listened to when he cries. They say when babies turn 4-6 mo, is the best time to start sleep training them.
JMO