Line 1 - MAS-AJJ-JTJ-SA-ED-CBE :
This route was Broad Gauge right from the inception and originated in RPM. The line was laid in 1862 and legendary Beypore exp/Fast Passenger ( Current Mangalore mail ) was introduced.
In 1982 AVD - AJJ was electrified.
1990 AJJ-JTJ/RU were electrfied.
1995...
more... JTJ-ED was electrifed.
1997 ED-CBE-SRR was electrified.
BG EMU Suburban Services were operated till Avadi/Villivakkam areas from Moore Market Complex.
Line 2 - MS- VM- VRI- TPJ- DG- MDU is the chord line:
It is the newest among the three you have mentioned. The construction began in 1927 and completed in 1929. Earlier all the trains from South TN were via Main line. This Chord line is 40 km shorter than the main line.
Line 3 - MS - VM - MV - TPJ :
This is the Main line . the construction began somewhere in 1870's from Mayiladuthurai. The entire stretch was completed in 1880. This route has many branch lines.
Line 2 and 3 were MG. The demarcation of Main line and chord line starts from Villupuram to Trichy. It was electrified between VM to MS even in MG era. The Suburban EMU services were started as early as 1932.