This is a tough one - 1975 is definitely one of my favourite years for music, as a lot of my favourite artists were at or near their peaks and as a result the year produced many great songs and albums. My three choices were:
Kashmir - Led Zeppelin
Wish You Were Here - Pink Floyd
Rhiannon - Fleetwood Mac
I found the first two very easy to choose - Wish You Were Here was an obvious pick for me as the title track from my favourite album (not just my favourite from 1975, but my all-time favourite) and Kashmir is a standout even amongst the many highlights from Physical Graffiti. the Rhiannon is my favourite track from Fleetwood Mac's self-titled album, an excellent album which tends to be overshadowed by Rumours but is a great listen in its own right. I'll give an honourable mention to Hurricane by Bob Dylan, although had Tangled Up In Blue (also released in 1975 I believe) been included in this poll I would probably have included it in my top three. Cortez The Killer by Neil Young is another classic song from this year that would have been a serious contender for a top three placing had it been included.