water filter plumbing question

[Alex]

Hi,

I have a puretec water filter that has 3 plastic pipes going to the
faucet. One of the plastic pipes goes to the drain, one delivers water
and I am not sure about the third one.

My plumber has expressed concern that if there was any blockage in the
drain the water would backfill up the plastic pipe, through the faucet
and back into the water purifiers holding tank.

So my first question is, why does this filter have three plastic pipes
going to it instead of one.

My second question is, is my plumber's concern about water backfilling
up the plastic pipe is correct?

Thanks

Alex

[Del Cecchi]

Does the code allow devices to be connected to both drain and water
system? You need to check the documentation that came with your
treatment system.