Electronic – Inductive energy when 12V relay coil is released

Every relay has a life expectancy... they are not infinite. You are saying that at 3 hertz you activate it a few million times, that is a lot of activations for a relay.

As you have not provided a part number, for illustrative purposes, I have used a generic one. The datasheet of your relay should have a graph like this one...

As you can see, in this relay particularly, the maximum life operations it can have no more than a million and a half operations...

That type of failure is normal in relays because the contact material starts wearing away with normal operation, and if you operate at the maximum rated load, as you can see in the table, the relay will have no more than 30,000 operating cycles.

I hope this information is clear enough.

The coil is not a "high voltage source", it's a current source, with the initial current being equal to the steady-state "on" current.

This only creates high voltages when the circuit impedance is high, such as when the driving transistor cuts off. If a resistor is placed in parallel with the coil, then the peak voltage across the coil will be limited to I_coil multiplied by the resistor value. Note that the voltage across the switching device will reach a value that is the power supply voltage plus the peak coil voltage.

The only reason that this method isn't more commonly used (than, say, the diode) is that the resistor also dissipates power for the entire time that the coil is on. Adding a diode in series with the resistor fixes this problem. And having the resistor (rather than just a diode) provides a faster turn-off time than the diode alone would give you.