Dual or Tri-axle Stepdeck trailers. Which is better??

All depends on the axle spacing.

 

Fontaine had a tri. that center could be lifted to make 10ft. spread or rear one lifted to be OK with 53ft. in CA.

 

My low pro steel trail king weighs right around 15

 

depending on the factors, but typically, that 3rd axle only gives you an extra 2500 pounds to haul. i think i figured the math to be about 1000 pounds for the axle. so basically, you can haul 1500 pounds more of load weight.

the 10 ft spread can haul 40,000. 3 axles take it up to 42,500

 

The 42,500 legal tri-axle weight in many states is on a 10' minimum front to rear axle spread(60" from axle to axle to axle), not 9'

Yes, all provinces in canada recognize tri-axles. They also allow more weight than in the US, so the additional tare weight is negligent up here. Check jurisdictional regs, but most of western Canada allows 24,000kgs (52,910lbs) as legal axle weight on a 10' tri group. Make sure you're registered for the GVW.
A three axle tractor with a 10' tri trailer is good for 46,500kgs (102,515lbs) legal.
Steering: 5,500kgs = 12,125lbs
Drives: 17,000kgs = 37,480lbs
Tri-trailer: 24,000kgs = 52,910lbs

 

You may get better info in the Flatbed Forum over HH, as there is probably more freight haulers there than here...
***caveat***I'm going from memory here as I haven't dealt with this for over 7 years.

I have no idea where you came up with an extra 15,000lbs???
Yes, you can add approx 1000lbs to the tare (empty) weight for an additional axle.
*In the US this would bring you up to 42,500 on the tri over 40,000 for a 10' spread axle in some states (NOT ALL), this gives you an additional approx 1500lbs of payload figuring 1000lbs of additional tare. This also does not allow you to just go up to 88,500 GVW. Each state will have it's limits.
ie: from TX to AB (remember this is at least 6+ yr old info), TX only allows 80k; OK, was registered for 90k, never really got weighed or dealt with any authorities across the panhandle; CO allows up to 85k with divisible loads on state highways. Read that! That means state highways, NOT Interstates!; WY, was registered to 100+k and 42,500 on tri is legal there, so 88,500=green light; MT, was registered 100+k, again 42,500 is legal there. So, we used to take our chances and sneak out of TX. If we were 85 or less we trucked on, if we were between 85 & 88.5 we had to watch out in CO. I don't condone this activity, but it does happen. The names have been changed to protect the guilty.

What the axles will "support" and what the various jurisdictions will allow as a legal axle weight are two different things.
ie: I have a CA 9axle. 3 axle tractor with three sets of tandems. That's 12,000+(34,000*4)=148,000. However I have only been that light about 4-6 times over the last five years. I'm usually in the 170-185,000lbs range as I can permit up to 40 or 45,000lbs per tandem (jurisdictional dependent) as well as a heavy front end with wide base steering tires allowing me 16-20,000lbs (again, jurisdictional dependent)

**The additional weight support from 40 to 60k (on a tri-group), is for one piece, non-divisible permit for over weight loads, then yes you can haul an additional 19,000lbs (20k less the 1000 higher tare) over a tandem or spread axle.

*this is jurisdictional dependent. I know Mt, Wy, Mn, & several others allow 42,500 on tri, but they must be 10 overall spread
**this is also jurisdictional dependent. The various states have different allowable limits. Ia will only allow 40k on a tandem, where other states around them allow 45k. I believe Oh also still limits tri-groups to 55k max (old info). You have to know your jurisdictions & your equipment and verify compliance before you make some of these decisions.
You are correct, none of the 4 western Canadian provinces recognize or allow spread axle trailers, so either keep the center deployed while up there or like you suggested make the rear the lift able one.