Definitely bring the -3 bag.  The hut is warm and has a heater.  You would be way too hot in the other bag.  If it is really cold you could sleep in your clothes, puffy jacket etc.

Thanks John.  I was leaning that way too but just wanted to be sure.  Beyond Asulkan, are other huts unheated (i.e. wood stove that would go out overnight) such that you'd want something warmer if I were to go to one like that on a different trip?

I always bring a zero bag, even for huts in the rockies where it is typically much cooler than Rogers pass (and even for huts with no woodstove/heater).  you can always wear all your clothes to bed but that is rarely necessary.  it is always heats up once people are inside.  my wife gets colder easier than me but the most she ever brings is a -7.  

What John said. Cuts down on weight..and almost always you are wearing long undies to sleep in if for some reason the night is cold inside.

Cannot stress enough to change your shirt and socks to a fresh/ dry pair when you arrive. Hang the shirt you wore during the day to breathe. I can usually stretch two days out of 1 shirt this way. Socks... no so much

Thanks for the insights guys.  Yea changing in to dry kit is always key, nothing feels better, and will keep you warmer.  Drying out the damp stuff is always a solid too.  

Good advice from JohnP but always keep in the back of your mind that on some hut trips you may not find the hut as planned. The Asulkan is impossible to miss as there is always a skin track direct to the front door and it is seldom fogged-out. If you are heading to other huts with less traffic then spending a night out could be a reality. Because of this I alwasy carry a -10 bag on hut trips that are new to me, remote, or not visitied as much. Also, the same goes for other huts with questionable heat sources. Spent a weekend at Grassy Hut in the Bonningtons a few years back when the wood stove was not air tight so we froze each night, glad I had my -10 bag with me! Plenty of options to choose from.

I have been to Asulkan a couple times for 5+ day trips. My -7 bag was more then sufficient, I wished I had something smaller to pack actually. It can be a bit of a taxing ascent depending on how efficient you pack. Awesome hut! Really easy to dry your stuff out. Watch out for the pine martin... he almost got our bolognese.. that's right, we brought up bolognese.