Do you have inductance in line with the current you wish to break? If not then you should not have any significant higher voltage generated by the opening of the relay. If there was any concern from stray inductance, a TVS should take care of it.
As far as the current being broken. While the relay may have a 3 amp rating it would be desirable to decrease the amount of damage to the surface from the arcing. And the assumption that because you are under the contact rating everything is OK is probably true, for a while.
I think there is good reason to go further to minimise any wear that would occur by not going further. My assumption is that you are using a single cell lithium battery. Voltage from 4.2 volts down to around 3 volts. Any voltage drop you have is a lot more significant at these lower voltages compared to switching 100+ voltages. Most relays are not specified to work at the low voltages you will be using. There often is a minimum voltage and a minimum current level specified.
Because you are switching DC current that means that you will have metal migration of the contacts in one direction. With AC the migration tends to balance out with random switching.
It is desirable for the opening of the relay contacts to happen as fast as possible to break the arc. You have stated that you know about reverse polarity diodes across relay coils to control the back emf pulse when the field collapses. While it is a cheap and easy solution, it is far from ideal. The diode will cause the magnetic field to collapse slowly. (relatively speaking) The switch contacts will also open slowly as a result of the slow decay. This will extend the time of the arcing and that is not desirable.
Automotive applications are mostly 12 volts dc. The switched currents are almost always dc. One of the things they do to decrease the switching time, the arc time, is to use resistors across the coils to absorb the back emf pulse. They balance the resistor's value with the coil current in order to get a faster decay time with minimal flyback voltage.
Take a look at these two links for further explanation.
On a relay coil, why use an MOV instead of flyback diode?
Why don't relays incorporate flyback diodes?