Most shows are filmed in 1080i so I suspect TV3 like many broadcasters are choosing to simply retransmit that rather than investing in an expensive deinterlacer. The fact that virtually all TV's sold these days are flat panels which are a progressive scan screen and not an interlaced picture like a CRT - interlaced video formats should be made obsolete by now!
Actually IIRC most US network shows if shot digitally in HD are shot in 1080p. They are then downconverted for transmission. That means when they do a BR or HD-DVD release they can issue it in 1080P. Also many are still shot on film, whether Super15 or 35mm and you can scan whatever format you want from that medium.
Given the US networks use both 1080i and 720P (from memory ABC, ESPN and Fox are 720p) while CBS and NBC are 1080i, I would guess TV3 could get whatever format they want to broadcast.