Not easy. Not easy at all...
3m x 3m (square) means same standing waves across and along the room.
You'll have a very pronounced ring around 114Hz and 228Hz. If the ceiling is also at 3m ish thing will be even worse.
Dealing with these low frequencies needs thick absorbers - 75mm spaced 75mm off the walls.
Any irregular surfaces for dispersion will help with the mids and highs but a large low frequency peak like you have is difficult to stop.
2m x 1m x 75mm hard fibreglass panels or Rockwool filled frames (cheaper option) covering most of the walls with corner bass traps are called for but they would take 300mm off the room dimensions.
https://www.studiospares.com/acoustics/acoustic-panels/primacoustic-broadband-panel-beveled-24-x-48-x-3-grey_464772.htm or similar
Tuned membrane absorbers matched to the room node(s) can also work.
Use Audacity to generate pink noise and put this through your speakers.
A decent LDC mic at the listening position and Spectralissime Spectrum Analyzer will show where the big peaks and dips are and their amplitude.