"can't imagine why anyone would want three strings on one note "

Doing some meditating a while ago, (having studied Music Theory...) and I achieved a 'point of enlightenment' about 'Modes' and stringed instruments...

Many people think that the lowest pitched string sets the 'home key'. Wrong. Many 'Modes' require a drop down to a fifth below the Tonic.

So the Second lowest pitched string defines the instrument natural 'Home Key' - a very good reason WHY the tiple is strung with 3 strings for the Second lowest pitch string.

Of course, there is no reason why the lowest pitch string cannot be used as the Tonic - but you can only then play certain 'Modes'.

Now as to WHY the next lowest pitched string is also tripled - for most tunes in one of the 'Modes' - you will usually need to play the melody on TWO strings ...

So - if an instrument 'grows up' in a culture that plays a certain style of music - it will reflect that. If others in other cultures 'borrow' the instrument, they may not understand the original purpose.