Services are big business, as you could tell because Apple had a whole press conference devoted to all of its new digital-only products. Amazon is the same, and it's always trying to entice people into signing up for stuff by offering free trials. The latest is another opportunity to get three free months of Amazon Music Unlimited, provided you haven't used the service before.
Of course Amazon already offers a 30 day free Music Unlimited trial for new customers, so really you're only getting an extra two months of free access. But hey, free stuff is free stuff, and all you need to do is make sure you cancel your subscription before the three months are up. If you forget you'll find £10 a month will vanish from your bank account until you get round to it. Unless you're a Prime member, in which case only £8 will vanish.
Unfortunately this extended free trial doesn't apply to existing Music Unlimited customers, or people who've redeemed a free trial in the past. Naturally you're not allowed try and redeem this offer more than once, and you can't combine it with any other offers -not that there are any more to redeem right now.
The deal is valid right now, and will be hanging around until 19th April. So get yourself sorted out before then, lest you miss the chance to not paying for access to music.