Is it safe to use AC power adapter of 6.7A or 4.62A instead of 3.34A

The given electric current ('amperes') of each charger is the maximum current which it can provide to your laptop. If your laptop needs less electric current then it is fine. However, if you would use the 65W charger on a height-performance laptop, which consumes more than than 3.34 A, it could overheat.

The voltage of the charger is important for your comparison. The voltage of all three is 19.5 V (voltage * electric current = power). Therefore, it is fine to use any of the other chargers.

I currently run a Dell Latitude E7450 with a Dell Latitude D series charger with higher power (cannot look which exactly) and it works fine.

edit: I assume that you are writing about alternative chargers that are produced by Dell?!

It's the voltage that hurts; the laptop will draw whatever amperage it needs, as long as the amperage is the same as or more than the required amperage (if the lappy docs don't tell you what amperage it wants, look up Ohm's Law).

If the bigger tranny is for using the laptop in its Dell docking station (which will be the same voltage, but draw more power), it's certainly safe to use.