Reactivity order of pyrrole, pyridine and indole with respect to electrophilic aromatic substitution is:

Pyrrole < indole="" />< />

Explanation:

1. Pyrrole:
Pyrrole has a highly delocalized electron cloud due to the presence of a nitrogen atom in the ring. This makes it less reactive towards electrophilic substitution reactions. The lone pair of electrons on the nitrogen atom is involved in the aromaticity of the ring, reducing the electron density on the ring. This decreases the electron density at the ortho and para positions making it difficult for electrophiles to attack at these positions. Hence, pyrrole is the least reactive towards electrophilic substitution reactions.

2. Indole:
Indole has an electron-rich pyrrole ring fused to a benzene ring. The nitrogen atom in the pyrrole ring has a lone pair of electrons that can participate in resonance with the π-electrons of the benzene ring. This increases the electron density on the benzene ring, making it more reactive towards electrophilic substitution reactions than pyrrole. The electron density is highest at the 3-position of the indole ring due to the presence of the nitrogen atom. Hence, indole is more reactive than pyrrole but less reactive than pyridine towards electrophilic substitution reactions.

3. Pyridine:
Pyridine has a nitrogen atom in the ring which is more electronegative than carbon. This makes the carbon atoms in the ring electron-deficient, resulting in a higher electron density at the ortho and para positions. This makes pyridine highly reactive towards electrophilic substitution reactions. The lone pair of electrons on the nitrogen atom is not involved in the aromaticity of the ring, making it available for reaction with electrophiles. Hence, pyridine is the most reactive towards electrophilic substitution reactions among the three compounds.

In conclusion, the reactivity order of pyrrole, indole, and pyridine towards electrophilic substitution reactions is Pyrrole < indole="" />< pyridine.="" />