The numbering convention for relays will tell you that 30 is common, and 87 NO, 87a NC. 85 and 86 are the coil.
I'll confess that I've never heard of a relay being "too big" except in terms of unnecessary current capability.
The question here is, why is the main relay coil getting enough current to keep it energised, when the ignition is turned off?
The fact that you're measuring 3 volts there suggests that either a semiconductor is failing/has failed and is allowing current through when it shouldn't, or that there's a bad earth somewhere.
Bad earths in cars are a very common cause of electrical problems, often showing up at this time of year (in the Northern hemisphere) when the weather has been wet, causing corrosion to accelerate.
A can of WD40 or something similar and a bit of time making sure any connections to the chassis are good will probably fix it, but if it's a duff semi, that's a job for an experienced auto electrician.