I may be wrong on this, but that sort of three jacks in parallel tap will work only on a half-duplex connection. Since most networks use switches nowadays, the links are usually all full-duplex. So the tap won't work.

An ethernet hub is the usual way to do a "tap", and by its nature is half-duplex.

Edit: you could try forcing all three ends of the connection to use half-duplex, and turn off auto-negotiation -- then the tap might actually work.

Cheers