Dive gear bag with Etihad airways

[Lyno]

So I'm flying out to Thailand next month and have read that Etihad have a 15kg bag for scuba equipment free of charge. I was just wondering if this means I can bring three bags (normal hold, scuba hold, and carry on) without paying any extra. Also does anybody know if you have to tell them in advance?
I tried phoning Etihad to ask directly but over half an hour on hold for an expensive number I had to hang up.
Thanks!