The 2015 edition was free to play for a limited time only. So if you got it during the free play (not free own) period, it will still be in your Steam library and enabled. If you got it during that period, then tried to play it and are asked to pay $9.99, that means you got it during the other promotion in which it was free for a period of time only. That being the case, you would have to buy it. Being in your Steam library doesn't mean you can play it. That's not how Steam works. e.g. during free play periods, when the period expires, the title can still be in your library even if you don't own it.
If you try to play it, and youcan, then you own it. And you only need to buy the DLC if you want 3.0 and future updates.
If you try to play it, and youcan't, then you don't own it. Then you'd have to buy both UCCE + the DLC. There's a 20% off bundle of both.