safe to use a different charger?

Your 3A charger would be overloaded. You need a charger capable of producing 6A, or more (never less).

No, it isn't safe.

i beleive it was Arther C Clark who proposed that "any sufficiently advanced technology is indistinguishable from magic"

probably not but it might overheat, melt, catch on fire, burn down you house...

i suspect that the high current rating is so that it can easily operate the computer, peripheral devices, and charge the battery at the same time.

i suspect that internal to the computer, a charging circuit will limit the current to something safe for the battery pack. (but that's just a guess)

it is a real good idea it use the mfr recommended power supply (or at least something with near identical ratings)

a lot of times when making an electronic device work to hard the magic smoke takes a permanent tea break and the device wont function again until you take it to the medicine man and have it blessed :)

If you're just using it as a charger, it may do fine if the battery is not fully discharged. It is usually difficult to determine the battery's state of charge and, as has been said, overloading the charger would best be avoided.

Having said that, I'm a little surprised by the power rating of the charger. Seems a little high for a laptop. It also seems a little high for a laptop battery charger.

Chuck