DOT regs for Canadian based carrier

I would set up 2 fleets and just put the one truck under it. You will have irp plates for that one and just Ontario plates for the others (assuming your operation now only travels in Ontario).
Only the drivers that go stateside will need to be pre drug tested and also be enrolled in a random drug testing program. It would probably be a good idea to have them all in the program but legally I don't see why you would be required to. As a new carrier (new dot numbers) you will have to do a new entrant audit after your authority is granted and usually they contact you within 24 months. They may ask for proof this is the only driver that is operating in the USA, you can provide trip records and show this is the only unit licenced for it and he is the only one that drives it. He will also want to see the first drug test date and time and a pass before the driver enters USA for first time. You may want to look up new entrant audit threads. You only need dot numbers on the side of the unit(s) that are going to the USA. I think it would be a good idea to leave the dot numbers off of the trucks that do not go there. No numbers on a truck hauling into USA is an excellent way to get attention. Goes to further your claim you are only operating the one unit internationally.
@not4hire is the regulation encyclopedia around here.

 

Your business will be the same. What I mean is when you go to buy your plates at the irp office you have Fleet 1 which will consist of plates for just operating in Ontario only. Probably look like
AB 12345. Then you will have Fleet 2 which will have the 3rd truck and will have irp plates that are similar to 102 3AB. And the cab card will have all states and registered weights listed. I believe the cab card also shows on it which fleet the particular truck is registered to. You are allowed up to 3 fleets (may have changed). It is exactly for the purpose of an operation like you have.

This may have changed with the irp transition to frp. The difference now is you just have every state listed on cab card rather than the ones you figured you would operate on. The idea is so each state gets its share of plate fees. So if you operated in Manitoba5% and Ontario95% but manitoba plate was 500 and ontario plate is 3000 you pay 5% of 500 + 95% of 3000 making it equal regardless of where you operate. This also brings to mind you will need to keep detailed trip reports including mileage driven in each jurisdiction. That's what you will use at renewal time and that's how they total up what you will pay for your irp (frp now) plate for that truck.

 

Can't really improve on what @gokiddogo has already posted.

As he mentioned, any truck(s) and driver(s) that run into the U.S. (even one time) will have to be fully compliant with all U.S. regulations, which includes the appropriate running authority and driver(s) being drug-tested. Also note that as of December 17, you will have to have an electronic logging device (ELD) on the U.S. truck(s) unless it/they fall under an exemption.

Although @gokiddogo mentioned that the separation of fleets can be by way of vehicle registration only, you may want to consider a separate corporation to provide an additional legal "firewall" from the rest of your business. You should get proper counsel regarding the pros and cons of doing that.

 

Should add:
Depending on how much business you may be shipping or receiving from USA you may be better off to hire it done. There really are a lot of regulations and extra paperwork and just general b.s. if it is only for a small amount of work it may not be worth the effort.

There is also the border crossing stuff. You will need a SCAC code as well as a carrier code. SCAC = paps for going to usa. Carrier code = pars for going USA to Canada. Relatively simple to apply for the identifiers and corresponding stickers (not necessary but are convenient). Also setting up with border connect in order to input your ace/aci into the system and create your eManifest in order to cross the border. I pay a service $8 per shipment in order to forward paperwork to customs brokers, and input my ace/aci stuff. They also keep track of (customs) brokers and light a fire under the appropriate ### as needed. Worth the money for me.

 

Do you already have a quote for going state-side? There is usually a pretty hefty premium to go across the border.