This probably means you and your friend have different computers. Some computers have built-in hardware limitations that prevent users from pressing three or more keys at once. Unfortunately there's not a whole lot we can do about this, so if you want to play chords we suggest you get yourself a USB keyboard. It doesn't have to cost a lot of money, and it will probably be a lot cheaper than buying a new computer :)
Here are some examples of USB MIDI keyboards that work great with Soundtrap.