I've implemented ΣΔ Modulator using Xschem. I works fine upto 2nd order. When i add third order module, it shows warning "Source stepping failed" and simulation stucks. Help me out Thanks & Regards

Having a bias current source shared between three blocks looks odd. Is this intended?

Yes, it was intended. Same issue is faced even if i use separate biasing.

Try ramping up the supply voltage from zero to the intended voltage over the first microsecond or so. Use a pwl source.