mm i finally put my hand in a old device and yes is happening what you reported only play twice and some mono sounds are not even played.
but is entirely an internal problem with soundpool, checking not happens from lollipop
just for test, set this sound to be played from disk and see if they are played 3 times at your side.
argh forgot this seems to be an old known issue using soundpool until lollipop
you may use play from disk to avoid this issue, until I found an easy way to implement this without affecting others