Fasts for two nights have been ordained for Vaishyas and Shudras (on certain special occasions). Fasts for three nights, however, have not been ordained for them by persons knowing duties.
चतुर्थभक्तक्षपणं वैश्ये शूद्रे विधीयते। त्रिरात्रं न तु धर्मज्ञैर्विहितं धर्मदर्शिभिः॥