Non sinusoide to sinusoide
A 3 kW inverter with a true sine wave output is very expensive.  One alternative is a "modified sine wave" inverter.  You might already have one, but we don't know because you have not told us the maker and model number, or posted a datasheet.

A square wave has many harmonic frequencies, while a sine wave has only the fundamental frequency.  A low pass filter can remove some of the harmonic energy, making the resulting wave more similar to a sine wave.  However, at your voltage and current levels the filter components will be very large and get hot.

The first step is to look at what you have now. Where are you located?