You know when you go to fill the tubing with insulin? Your pump will just keep pushing until it hits the reservoir and you’re finished. Same procedure for using the 3ml just with a shorter reservoir. It will twist and lock in the same way. Try it. Your pump just won’t read you as having a full reservoir.
I have never used 1.8 reservoirs so I am not a good source. However as others have said I do not think it ill be an issue. The pump will rewind to accommodate the 3ml reservoir and then you will advance it until it meets resistance. From there it should behave normally.
The down side will be it will take a bit more time to get your pump primed. The upside, you will save insulin.
It seems like a more than even tradeoff. Oh and I found the reservoirs in the medtronic diabetes shop.