Mine died in 4b during a -34C winter. I don't think I have ever seen it labeled as zone 3 anywhere in Canada. Better off trying some hardy apricots if you want to experiment with stone fruit.

http://www.mofga.org/Publications/MaineOrganicFarmerGardener/Summer2000/Peaches/tabid/2315/Default.aspx

"The hardiest cultivars are rated as zone 4 to 5 in hardiness. ‘Reliance’ peach, a medium-sized, yellow-fleshed peach of good quality, developed by Elwin Meader at UNH, is the hardiest variety available, considered zone 4 in the right location."

In zone 4 peaches need winter protection or very sheltered spot. They will survive Though

I have grown Reliance in SE WI for about 16 years now. I am in northern Zone 5. They do well for me except 4 winters ago (?) when we got a week of -18. The following spring I lost most of my peach trees. I kept one that was severely damaged with winter injury and it came back with new growth and survived.

Reliance grown at a friend's place in WI (zone 4) has survived but due to the colder winters they are lucky to get 15-20 peaches off the tree. The same year mine is covered with fruit. I suspect they loose a lot more fruit buds with the colder winter temps than what I get.

I have been to Northern Wyoming and would think your winter temperatures will make it challenging to get peaches reliably. Not saying not possible but try to place the tree in a courtyard, east side of a building ect to give it some protection. Any sort of protected site or area with a better microclimate would help.

I have friends in NW WI who know they cannot reliably grow peaches. They ended up putting them into giant tubs and putting the tubs into an insulated but unheated building for the winter. I recall their peaches (PF24C) survived a few years until they outgrew the pot. They did get some nice tasty peaches off the tree. A temporary crop but perhaps in some cold climates that is the best way to go.

I lost Reliance to -37°, and -31°F. I think at about -28° you will see damage to the tree. It seems to be in the trunk. I might try wrapping the trunks over winter in bubble wrap in the future.
I
encourage you to try to grow peaches (as long as it is not for commercial purposes), but you may need to replant every
5-years or so depending on the weather. I probably have had a crop
3-years out of 10 owing to the unusually cold weather two years. I think
3-years out of 5 would be more ordinary.

Many of my Reliance and Contender peach trees have survived for 5 winters here in Zone 4B/5A Central Maine. I have lost some due to canker issues and a few due to sudden peach tree death. My orchard seems to have stabilized over the past few years though with younger trees doing much better than they did when I first started out.